These are war strategies that I have developed during two long wars in which unexpectedly backfired on me. These have only really been tested on chieftain, where the AI is comparably dumb to other levels. So they might not work too well. And these are really towards modern warfare.
1) Whatever you do, do it quick: Long wars can drag you into war weariness if you don’t allow the government to manage citizen moods and even if you do that it can lead to starvation. Allowing the war to progress on also gives the AI the chance to pillage your roads, maybe starving out your population. If the war is long there’ll be more resistance, resentment, and hardship. Though Democracy has the highest war weariness, if you let the governor manage moods it won’t be a problem.
2) Decide on the objective: This is really important. If you expect to keep all captured cities, then don’t mobilize; if you do, make the war quick. Done wrong, mobilization through a long war cripples the ability to keep your cities; you can’t build any culture improvements. I was once tied into a long war (doesn’t the stupid AI ever die? I had that ‘settler in a ship’ thing with France) and I was increasingly fearful that a city could culture flip. Mobilization backfired badly that time, and it paid to decide on an objective the next time I went to war.
3) Be wary of the AI: If you sign mutual protection, get the AI in a war, and they like the enemy, they’ll end the mutual protection and get out of the war. Be careful too about refusing embargos or alliances, because they’ll sign an alliance or embargo against you if you decline. An entanglement of alliances, protection pacts, and trade embargos can involuntarily tie you into a one-on-four scenario. Also make sure that the enemy has no MPP ties or lots of trades going on. The AI can be double-dealing and nifty. If you don’t watch it, you could be at war with 3-4 civs at once. Disaster erupted when I was at war with the Americans; China, who had an MPP with me, signed an MPP with America, and I was at war with them.
4) Seize on the AI’s weaknesses: The AI doesn’t efficiently strategize or care about their cities. See what the AI does, and then defeat it. In one war the AI kept on sending hordes of Longbowmen and Calvary across my borders and kept on pillaging. I parked MAs along the border and systematically destroyed stack after stack after stack of troops. About cities: Far-flung cities (you know, the one that they trespass in your territory ten million times to exploit a tiny hole) generally have no culture improvements and guarded poorly. Capture and raze those cities first.
5) Take advantage of zone of control: Mechanized Infantry and Modern Armor are two of the best examples on zone of control; both of them have zone of control. If you’re conducting modern warfare, park lots of these units along borders; when units come to invade, they’ll automatically shoot, eliminating large stacks of enemy units and making it less likely that you’ll lose those units. This is only effective when the blasted AI is dumb enough to keep on sending unit after unit to the same spot. ANY unit that has a zone of control is a helpful unit. AGEIS cruisers, radar artillery, and fighters – they can fortify your defense capability.
6) Know what units can destroy what: Don’t send out horsemen if they can get beat by spearmen. Infantry vs. Riflemen is slaughter for both sides of the battle. Submarines are good for spotting AI subs. Cruise missiles can destroy conscript units. Radar Artillery can liquidate a seemingly hard city takeover to easiness. It seems stupid, but it can save a lot, in failures, unit destruction, and defeats.
7) Don’t underestimate power: Transports should never be unescorted in the world of battleships and destroyers. An injured Modern Armor can’t beat a stack of healthy longbowmen so easily. Cruise missiles can achieve every time what Radar Artilleries can’t. One-unit cities can be recaptured. Pitting units that have similar A/D/M ratings can end up in disaster for you (Riflemen vs. Infantry).
8) Count on defeats: It’s hard, but it’s going to happen. Modern Armor can fall. Cities can be captured and razed. Make sure that you are constantly building units to replenish lost ones or to better defend cities. And you’re sleazy with dealing with the AI. Sometimes the AI will suddenly will send in hordes of troops like they’ve only started to fight.
9) Expect the unexpected: This is crucial to fighting in a war. Units may unexpectedly fall, cities may be captured. The AI could be underestimated and have more units than you think. War may never go as it is planned. It’s crucial to remember that.