I have noticed in most of my games that there seems to be 3 stages at which it is best to go to war.
The first stage is shortly after Iron Working and thus Swordsmen are on the scene. I am able (at least on Regent) to get I.W. pretty soon after, if not at the same time or before, the AI's through trading. Also, by this time, I should have developed my cities with a few road/mine combos and barracks to be able to churn out swordsmen pretty quick in the core cities. Faster than the AI can as the AI has usually neglected city development except for a few irrigated tiles. This allows me to wage a short but successful war to take 3 or so cities, thus crippling the AI just enough to remove it as a serious threat for a while and simultaneously boost my own empire (I, of course, attack for resources and lux's, so if a lux is badly placed in relation to my palace or FP I will let an AI settle there and develope it a bit and then take it away later on)
The second stage is Cavalry. Like the swordsman, the cavalry is a huge step up in attack strength (not to mention movement) from anything else around at the time. In my opinion Knights don't come close, being only slightly better than swordsmen. (In fact I would rate a veteran swordsman about equal to a regular knight...well...almost!). At this stage, war can either be aimed at grabbing more strategic land, or just crippling your strong neighbours. Both the first stage and second stage I prefer to keep the wars short and get back on decent terms with the AI's ASAP. I'm usually at cavalry (Military Tradition) about 8 turns or so after the top AI civs (the AI seems very reluctant to trade MT; something to do with my stage one foray perchance??), but the AI still seems unable to build a big or effective (veterans and a few defenders) army.
The third stage is from tanks onwards. By now I should (remember, this is only Regent) be ahead of all but the top AI Civ in Tech so my poorer neighbours are defending against my tanks with Cavalry or musketmen/riflemen. At this stage I start churning out huge amounts of tanks and quite a few mech. inf. aswell and demolish a few weaker civs, grab their land and expand. Keeping in mind that the top civ is my unltimate enemy I make sure that my economy is still growing. Then, when I get Modern Armour, which should be before even the top civ if I'm careful (the AI seems to regard Synth. Fibers as quite a low priority), it's all on! This stage's war is long and bloody. ..and bloody good fun (when I'm about the put my long-conceived war plan into first gear my heart starts beating faster!)
Also, in my last game, I tried something which someone else had suggested: cut off and lay siege to the enemy capital. Take over all their cities around the capital, and destroy all improvements around the capital, but don't attack the capital (unless it's with planes or artillery so you don't capture it). This means that their Palace is effectively kept prisoner and there are no cities to take advantage of the Palace benefits. With a stack of 5 or so Mech Inf and a few Mod armor hanging around, anything the capital builds and sends out is dispatched without so much as a twitch. It worked really well.
Basically, when I first get Swordsmen and then Cavalry I start beating the war drums and looking for a likely foe. (When I say "likely foe" I don't mean something honourable like an enemy who will put up a decent fight. I mean someone weak and pathetic that I can attack without fear of spilling my wine. )
The first stage is shortly after Iron Working and thus Swordsmen are on the scene. I am able (at least on Regent) to get I.W. pretty soon after, if not at the same time or before, the AI's through trading. Also, by this time, I should have developed my cities with a few road/mine combos and barracks to be able to churn out swordsmen pretty quick in the core cities. Faster than the AI can as the AI has usually neglected city development except for a few irrigated tiles. This allows me to wage a short but successful war to take 3 or so cities, thus crippling the AI just enough to remove it as a serious threat for a while and simultaneously boost my own empire (I, of course, attack for resources and lux's, so if a lux is badly placed in relation to my palace or FP I will let an AI settle there and develope it a bit and then take it away later on)
The second stage is Cavalry. Like the swordsman, the cavalry is a huge step up in attack strength (not to mention movement) from anything else around at the time. In my opinion Knights don't come close, being only slightly better than swordsmen. (In fact I would rate a veteran swordsman about equal to a regular knight...well...almost!). At this stage, war can either be aimed at grabbing more strategic land, or just crippling your strong neighbours. Both the first stage and second stage I prefer to keep the wars short and get back on decent terms with the AI's ASAP. I'm usually at cavalry (Military Tradition) about 8 turns or so after the top AI civs (the AI seems very reluctant to trade MT; something to do with my stage one foray perchance??), but the AI still seems unable to build a big or effective (veterans and a few defenders) army.
The third stage is from tanks onwards. By now I should (remember, this is only Regent) be ahead of all but the top AI Civ in Tech so my poorer neighbours are defending against my tanks with Cavalry or musketmen/riflemen. At this stage I start churning out huge amounts of tanks and quite a few mech. inf. aswell and demolish a few weaker civs, grab their land and expand. Keeping in mind that the top civ is my unltimate enemy I make sure that my economy is still growing. Then, when I get Modern Armour, which should be before even the top civ if I'm careful (the AI seems to regard Synth. Fibers as quite a low priority), it's all on! This stage's war is long and bloody. ..and bloody good fun (when I'm about the put my long-conceived war plan into first gear my heart starts beating faster!)
Also, in my last game, I tried something which someone else had suggested: cut off and lay siege to the enemy capital. Take over all their cities around the capital, and destroy all improvements around the capital, but don't attack the capital (unless it's with planes or artillery so you don't capture it). This means that their Palace is effectively kept prisoner and there are no cities to take advantage of the Palace benefits. With a stack of 5 or so Mech Inf and a few Mod armor hanging around, anything the capital builds and sends out is dispatched without so much as a twitch. It worked really well.
Basically, when I first get Swordsmen and then Cavalry I start beating the war drums and looking for a likely foe. (When I say "likely foe" I don't mean something honourable like an enemy who will put up a decent fight. I mean someone weak and pathetic that I can attack without fear of spilling my wine. )