>Combined arms was a concept understood at the time (Ming Dynasty in this 17th century battle) and had been used since the 400s BC (when crossbowmen were combined with archers, pikemen, halberdiers, etc in Warring States armies). The Ming Dynasty had pike formations that combined musketeers with pikemen and archers.

Military tactics and theory were not directly passed down throughout Chinese history. Many things related to mathematics, science, engineering, military strategy etc. were either rediscovered or reinvented hundreds of years later or not even known that the Chinese possessed such knowledge till the 20th century until studies by Sino Scholars. So just because there is an example from BC's, doesn't mean it was still actively analyzed in AD.

And the reason I said the Ming had no concept of combined arms was I meant they had no idea how to use musketeers and artillery with their infantry and cavalry. So firearms weren't very effective, which is why the Chinese never fully invested them in as opposed to the Japanese and Europeans who saw the huge advantage that firearms had and basically equipped their entire armies with. The Japanese actually used massed rotating volley fire combined with infantry and cavalry support to resist charges and push back against the enemy. The musketeers were used like how Napoleon used artillery and cavalry together. Break up the formation of the enemy with withering firepower and charge in with cavalry backed up by infantry. The Ming were defeated in almost every pitched battle against the Japanese in Korea and only pushed through due to the Japanese running out of supplies or retreating when in danger of being isolated by the Ming and Joseon forces. So yea, maybe some Chinese strategists were aware and implemented such tactics but clearly those ideas were not spread to the entire Ming military nor became uniform tactical theory across the country.

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>The 16th century Ming general Qi Jiguang even developed a quasi-pike formation called the Mandarin duck that combined shielded swordsmen + pikemen + ranged troops (muskets, archers, etc) + a guy with a weird polearm called the wolf's brush.

You're talking about the polearm that was like a bamboo branch that basically shoved in the enemy's face right? I've heard of this too but I'm assuming it was that one general's tactic and no one else used it. And it seems he mostly used it against Japanese pirates and rebels in the South. No idea how it would have fared against the Manchus or actual Japanese military units.

Impossible to know for sure but effective use of musketry by the Japanese made a huge impression on Korean and Ming soldiers during the Imjin Wars in Korea. Dedicated musketeer units were formed in both countries (although Ming most likely already possessed some musketeers even before the war), however Japanese muskets were works of art, well made by master sword blacksmiths and well appreciated. China and Korea tried to imitate the Japanese use of muskets but corruption ensured that most muskets were useless on the battlefield as they were not well maintained nor constantly practiced with. The worse thing for a soldier is to have an unreliable weapon.

Most likely rusted out muskets brought from some dinky storehouse with mishaped musket balls and poor quality gunpowder etc. Even well maintained muskets were notoriously inaccurate with soldiers firing over the heads of their enemies even during the American Civil War. So, I can imagine even worse issues with the Ming forces which were pretty much just cannon fodder by the 16th century. Doubt they had any proper musket training nor was combined arms a concept understood in China at the time.

Never quite understood why Chinese and Korean armor quality seemed to deteriorate by the 15th and 16th centuries to the point that normal foot soldiers barely wore any armor. Ive heard armor was rendered ineffective by guns but guns were expensive and most armies wouldnt have been equipped with them, especially the Manchus or Mongolians etc. Also, Japan still used high quality armor even though they spearheaded volley tactics in Asia at the time. Seems more like the institutions in both China and Korea were so eroded by corruption and incompetence that soldiers were regarded as expendable and thus werent provided with armor. I read that by the 18th century, most Qing brigandine armor didnt even have any real iron plates inserted between the cloth and was basically just cloth with metal studs for show.

Brigandine armor also seems to have become most popular sometime during the 14th to 15th century in East Asia, but brigandine also seems like the cheapest armor option between plate, chainmail, lamellar, and laminar. Ive wondered if doubling as a coat made it more popular or if it was just the cheapest option since its easier to maintain than the other armor types and less obvious if the metal plates were missing or of bad quality.

I've never been there but even from the pictures it looks way smaller than how its portrayed in Gladiator. In Gladiator, it looks like a football stadium.