I focus more on Western Europe during this time, but you are right to say that pitched battles were much less common than sieges. Pitched battles often resulted in the end of a campaign for one side. Whereas strategically sieging castles/cities gave broad command of an area, losing a pitched battle could effectively mean loss of the war as a whole in an instant. Capture of key figures like high nobles or the king himself could be the undoing of an army regardless of their number or quality. Unlike TV/Fantasy portrayals, in a feudal society a leader had to convince vassals to come to war with him. Famously, English kings struggled to get Northumbrian barons (northern England) to go to war in France because they saw very little gain for themselves being so far removed. Even more than just losing simple lives, losing a battle cost a king one of the most vital resources he had: confidence. Powerful nobles would often change sides or abandon the cause, sometimes right before a battle. For these reasons and many more, a methodical campaign of sieges was preferred.

To your question, the first thing a ruler needed was a cause. I think of Plantagenet kings like King John trying to reconquer their familial holdings in France. Kings were known to come up with wild reasons to go to war, many of which were over exaggerations or outright lies. But, there had to be a specific aim and a reason to fight (especially when warring against another Christian nation). The next thing they needed is the money to carry out a campaign. Funding would come from taxes levied on barons (the same Kings depended on to fight with them), the church, and even peasants through royal sheriffs and taxes on trade/products. Some kings ran wild with taxation, like charging Barons insane sums just to inherit their father's land and estates. The funds a king extracted would be needed for supplies, transport, and most importantly, mercenaries. When cash on hand ran out, many kings would borrow from other nations (like Italian bankers) who were basically speculating on the outcome. Many of these bankers lost their lunch when the king they financed lost his war.

The promise of plunder and a regular paycheck was very appealing to mercenary bands, and for that reason medieval nations rarely went to battle with only their own troops. Mercenaries were often times more loyal than nobles but were prone to running wild when the fighting was over. Sometimes they had to be physically driven out by the lords that hired them. Flemish mercenaries were widely hated by the English in particular.

I'm a little less well read on campaign planning as a whole, or maybe we just don't know how it was done in any great detail. Generally, the English would try to capture a castle along the coast that could serve as a hub for further attacks (see Harfleur). Supplies brought from home would only last so long, so it was commonplace for nearby villages to be raided of food and any valuables. These supplies were carried behind armies in what is known as a baggage train. Cart after cart of supplies was often a target for opposing armies, who could defeat an opponent without fighting them in the open by starving him.

Crop burning and general destruction was rampant as well. The English caused especially visceral damage to the French as a dedicated strategy during the Hundred Years' War, partly to try and force a pitched battle. Many losses in any campaign came from disease, especially dysentery (called bloody flux at the time) because siege camps were mostly disgusting places with cramped conditions and very little sanitation.

Victory or defeat was usually cemented by the signing of a treaty, which would have terms that could include the secession of land, money, and even marriages. Usually a country was brought to its knees in a flashpoint, like the capture of a vital position or massive defeat in a pitched battle. I think The Hundred Years War: The English in France 1337-1453 by Desmond Steward is a book you would like. This series of conflicts has fundamental tenets of warfare that I think apply broadly to any ruler going to war in the Middle Ages. Most of my answer has an English slant because that's the only area I feel comfortable answering for.

This is a pretty simplified rambling explanation, but I hope it at least points you in the right direction!

Not really familiar with the middle ages in terms of campaigns, but I have some familiarity with ancient warfare.

Sieges in general tended to be very costly during warfare and a time wasting avenue. Depending on how well fortified a settlement or a fortress can be, you could take months or even years and a siege would mostly end up just standing around a city waiting for something to happen. Even worse if that city is close to the coast and can be supplied by sea.

Imagine if there were at least 20-30 forts or even more well fortified cities, there were a lot of fortresses and cities you have to pass in order to advance deeper into enemy territory. Usually it isn’t recommended to leave enemy fortresses and cities behind the army as that will expose them to harassment from the sides or rear. Small skirmishes that slowly chip away an army’s ability to fight. Any foraging party would be ambushed and destroyed in small skirmishes. A couple hundred or thousand depending on the scenario.

You are mostly correct in saying that pitch battles don’t tend to happen a lot. Though I think that people have this misconception of pitch battles being too scarce.

For one, we’re relying on written sources most of the time in order to determine how much pitch battles were conducted. This means that we usually don’t get a clear picture as to how many times it really happens. Or how many that actually did happen occur. Some time periods remain obscure and have lack of written sources that clearly portray events accurately. Some sources go so far as to “mention” a battle but glossed over such events.

The late roman period have huge amount of sources in the 4th century, but the 5th century has
very few written records to depict this period. We have a general idea of what happened during this period of strife and unrest but times can be quite chaotic that there just aren’t that many written records. Archeological record is even harder to find. Some findings by historians in Europe tend to be in a different language. Roman artifacts can be found in many places in Europe. Not all of them are translated however and its usually time consuming to translate those works. There is also political quagmire to get those works and publish them.

That said pitch battles happens under specific circumstances. The enemy has a huge force and you need to overwhelm them while morale among your soldiers are high in order to capitalize on the fact that the enemy is grouped up together tightly and engage while they are all in one place instead of spread out over a large area in small parties.

For instance, we can use Julian’s campaign in gaul mentioned by Ammianus Marcellinus in the 4th century. The germanic tribe confederations of the Alemmani and Marcommani and several other confederations of combined tribes attacked Roman Gaul and completely overwhelmed the entire province. Sacking various cities and settlements. Some reaching quite close to Italy.

The breached frontier needed to be reclaimed. Gaul served as the primary recruitment drive of most of the Western Roman army since its administrative split with the east. The second one comprised of regions of Hispania and Iberia.

Germanic confederations of multiple tribes demonstrated that many of them were becoming much too strong and were getting more aggressive against Roman frontiers ever since the late 2nd century.

Julian’s move against the Germanic tribes is to capture back the region and bring it back under Roman control. Various minor battles were already occurring between Roman troops and germanic soldiers.

Julian led by example. A typical bookworm that was considered a decent Caesar by Constantius II to govern the west at the time. Constantius was a bit of a paranoid men and had murdered much of his family in a bitter power struggle to get the throne. He became Augustus of the the entire empire but allowed some degree of autonomy to Julian considering that he wasn’t much of a threat to Julian at the time.

Julian studied history and battle tactics of previous emperors and led successful campaigns against the germans using tactics of his predecessors. Influenced by the likes of Alexander III he inspired his troops but was hamstrung by internal corrupt bureaucracy. Several officers were suspected to be sent by Constantius to keep watch over the young Caesar. Several of them led their troops back to Mediolanum to their winter quarters and abandoned the campaign leaving Julian with a meager force of around 15,000 men.

King Chnodomar the leader of the germanic tribes sought to wipe out Julian’s force now that it had reduced to a smaller force. His forces supposedly numbered 30,000. He summoned back the raiding parties to group up. Julian was camped in one of the local abandoned cities and was informed of the presence of the german army nearby and wanted to avoid battle first as it was closing night time as his troops needed rest. His troops refused such an order and all clamored to engage Chnodomar arguing that now the enemy were all grouped up together, they could defeat them in one fell swoop ending the campaign while their morale were high. Julian’s officers also encouraged Julian to engage. Julian decisively defeated the germans at Strasbourg in 357 AD.

Several more conditions are needed to engage in a pitch battle. Accident. Sometimes both armies accidentally engaged one another. For example, foraging parties of both side engage in a minor battle by accident due to thick fog and huge ridge lines. They then send parties back to both camps respectively to inform their generals that they need help. Both generals send more and more to reinforce and then quickly form up both armies to engage. Such an event happened during the battle of Pydna between the Roman republic and the Macedonic kingdom.

Or sometimes the enemy are just too large in force and you need to destroy them before they became too huge. If a general avoids battle he can be seen as a coward from his troops and can be potentially assassinated for cowardice. Many soldiers defend their own frontier towns and had settled families in the region. To avoid battle is to put their families at risk. Large scale mutinies among troops who felt their families were ignored can be very dangerous. Indeed generals can be forced to fight in a pitch battles by their own troops. Local politics also play a part.

If the people are angered by the lack of governmental efforts to destroy the invaders destroying the countryside, they could rebel against their rulers who they saw as negligent in their duties to protect the lands from being destroyed. This is why scorched earth or delaying tactics by rulers are incredibly unpopular. Farms and towns destroyed. Food such as cattle are taken by the invaders. This can destroy a ruler’s reputation and paints him as someone who can’t protect their people. This is something the opposing army can take advantage of. This is why pitched battles can occur more quickly than what is typically portrayed. Pitched battles less commonly but are not super rare as one might think. Just not very common.

Really an excellent question. As noted above, it's a bit dependent on where in the world you're looking, and what kind of battles (ie, land or sea) and in what time periods. Generally speaking (pun intended, and I mean, in the broadest possible sense as well) a pitched battle was decisive for an entire war. They were very rare, and often consciously avoided due to the immense risk. Some of the most brilliant war leaders in history actually intentionally forced pitched engagements to quickly defeat logistically superior foes. Examples: Genghis Khan, Subotai, Alexander the Great, Hannibal Barca, and Cao Cao (who ironically got spanked in a decisive fight too). Siege warfare was the expectation in western europe and the near east for a long time. Taking castles meant taking land meant expanding logistical base. This pattern is broadly true in the far east as well, with the caveat that the Chinese, especially during the warring states period, skirmished and battled in very large (possibly exaggerated) numbers.

That is an interesting question. I would like to know more about it too.

1100-1300 covers most of the Crusades and the Mongol invasions of Europe.Local wars in Europe were not usually large but the Crusades were multi year affairs involving large armies, sieges and fleets. Large local conflicts could not be paid for by pillaging and looting small neighbors. Islamic armies of the period fielded similar large forces, many of African origin.

After reading multiple books on medieval warfare it would be overambitious to give any level of summary considering that the practice of warfare changed from century to region and from region to region, for instance, the Irish denied Anglo-Normans from pitch battles, preferring forest ambushes, making the conquest of Ireland slow and painful. In contrast to warfare in France where you would immediately meet a field army.

A lot of this is going to depend on where exactly you are, mind:

1. The Teutonic Knights, for example, spent most of their time fighting low-level skirmishes with pagans, often in winter, and the rest of their time sieging down forts whenever they had Crusaders come to help.

2. The Byzantines, by contrast, preferred to build bases of supply for their armies, and gradually moved them when planning an attack to reduce the logistical strain, and usually relied mostly on locals to defend territory.

3. The Hungarians (Pre-Mongol) were usually an exception, preferring to seek pitched battle against invading nomads, with little in the way of stone fortifications outside their western border. Against the Mongols, this proved a disaster.

4. While on topic, the Mongols preferred attacking the exposed population instead of sieging western stone forts (built tall to be difficult to storm, though this also made them far more vulnerable to later cannon than the earth-packed Chinese walls,) and the lack of wealth they got attacking Hungary this way was a big factor in why they didn't bother going further west.

5. The English preferred raiding the French over sieges, but this meant that anything lost to France was that much more difficult to take back. In many ways, the English had no choice but to do this, as gathering enough ships to move troops was very difficult without a royal navy.

Stinky unwashed people, fresh from bashing, cutting and slicing people apart, hand to hand.

Imagine WW1 trenches, wet, cold, disease ridden... Now add some unknown thing called ptsd from physically downing swaths of people, animals, things best left unseen... Now add Uber religion and superstition...a total lack of medical care....

Total hell on earth..

And I think you have the beginnings of an understanding.

Mostly raids against the civilian population (the English called these chevauchees in the Hundred Years War). The aim was both to damage the other side's resources and pressure the local nobility into defecting to your side. Each noble or town that came over expanded your zone of control and diminished that of the enemy. This involved lots of minor skirmishes, perhaps some attacks on fortified positions if these looked vulnerable, and possibly a siege against some key position. In western Europe 1100-1300 it involved lots of negotiation, intercession by third parties (the Pope, maybe the Emperor in the German lands, or the French King before 1200), assertions of right and so on.

I think people exaggerate how rare pitched battles were. In most major wars in medieval Europe, you see at last one pitched battle. That said pitched battle alone did not necessary decide the war as pitched battle was not war goal on it's own. Given most of the time war goal was control over territory and resources and territory and resources were usually protected by some kind of fortifications, you needed siege to reach your goal. So pitched battles were important as far as they either led to a successful sieges or prevented enemy from successful sieges. There are number of examples of military campaigns and pitched battles that did not result in successful sieges and control over territory/resources. Notorious example are English campaigns in France during Hundred's Years War. English often won major battles, sometimes in spectacular way, but they sometimes failed to transform those victories in to territorial gains and control.

So how would major military campaign of the Medieval era look like in Europe? Feudal lord, for example king would declare war over some (usually territorial claim). He would set up a war goal, usually town or fortification commanding that territory. He would call up to arms his subjects and set a place where troops would gather. This might take few months. Preparations would have been done in advance (could be as much as year or half a year in advance) to gather equipment and supplies (may include ships). Once assembled, leadership would be assigned and march routes planed. Then army would march to it's objective. On the way army would conduct raids and siege any fortifications and settlements that would pose obstacle towards objective and to secure lines of communication and supplies. Once at the objective, siege would begun.

Opposing side in the mean time would start assembling it's own forces, typically with a time lag of few months. If force assembled was deemed large enough to confront the attacker, army would march at the enemy with the goal of either preventing him from reaching his goal or lifting his siege of the objective if he already reached it. If nothing else intervened, armies would meet and fight pitched battle. If attacker lost, war would be over. If attacker won and his force was still strong enough to carry on to the objective, he would proceed with his goal. There was still the chance that he would fail his objective however as he might still have to fight the siege. There was also chance that even after victorious battle, losses would be such, that attacker would not have enough strength to capture the objective, in which case he would fail.

If assembled defending force was not deemed large enough to confront attacker in open battle, it could still try to frustrate attacker's siege of his objective. They could try to de-blockade siege town or fortress, reinforce it, raid enemy rear, set up ambushes, prevent him from foraging effectively or just simply pose threat by been present nearby.

In some instances defender might fail to assemble his force or assemble it on time and attacker might have already taken his goal. If defending side refused to accept such outcome, they may try to mount their own campaign later to recapture lost territory. Sometimes one or more years later.

Thanks to character of feudal mobilization and economy, feudal warfare was extremely indecisive. Meaning it was very hard to defeat enemy decisively on a strategic scale. Forces that could be mobilized at any moment in time were as a rule only fraction of actual military capacity of the country. Large part of feudal levy was immobile and incapable of leaving far from their homes because of the lack or transport and seasonal character of agriculture. And even more mobile part of levy composed of feudal lords, their retinues newer assembled in full when called. Therefore feudal armies were small in size relative to overall military capacity of the country and were restricted in time they could effectively spend on the campaign. Therefore even if such army was defeated in the field, it was always possible to assemble a new one over time. Add to that fact that movement and communications were slow, feudal wars tended to drag on and resemble pendulum: Side A would assemble force and attack side B. Since side B started to assemble it's force later and with a time lag, side A would have initial advantage pushing forward. However over time force of side A would diminish while side B would have assembled it's own force gaining advantage in turn. Rinse and repeat. Therefore large feudal wars tended to go forward and back for years with sides gaining and loosing momentum as they gathered their war resources, spend them, retreated back to recover, gathered their resources again and so on.

Because of above, lot of medieval warfare was actually raiding. In fact some military campaigns had raiding the territory as a goal, rather then capture of that territory. This was to some part also caused by character of motivation of Medieval soldier, for whom war booty was important part of warfare. Over time such raiding could diminish resources of the opposing side and could be more effective then any pitched battles.