Thank you for your suggestion

Can anyone recommend a book which compares and contrasts some of the major empires from world history? I'm hoping to find something that stretches back to the Roman Empire and takes in some of the Chinese empires, the Moghuls, the Ottomans, Austria-Hungary, the British Empire, and more besides. I'm particular interested in the "decline and fall" aspects (i.e. what were the events and conditions that led to the demise of the empire in question), but general histories are ok as well.

Andrew Roberts' excellent biography Napoleon the Great covers all the major campaigns at a "birdseye" level of detail. It's incredibly readable and of course as a biography it sets them into the context of Napoleon's life as a whole. It's not specifically focused on the military aspects, but each battle gets a fairly thorough write up, including lots of detail about troop numbers, equipment tactics, and so on. The withdrawal from Russia is done very well indeed. I think he could have gone into more detail about Waterloo, but overall I loved it and it's one of my most recommended history books.

(I suppose you might argue that the earlier campaigns are part of the French Revolutionary Wars rather than the Napoleonic Wars, but if you're interested in the period then I still think this is a great introduction).