Is there anyway I can stop an AI from pursuing a military victory, or do I have to break out the spears to stop him? Should I rush a world congress and try to impose sanctions, or would that take too much effort too late?
There are probably a million different ways, but most of them will fall into a few broad categories. I'll try to keep this applicable to vanilla Civ V, but also mention things in the expansions.
Somewhat direct routes to opposing their military victory include:
- Take their capital. This is the obvious one. Since Domination victory requires being the last civilization with your original capital, taking an AI civilization's original capital completely prevents a Domination victory from them, at least until they can retake it. If you yourself have a weak military, you can try to goad/bribe/otherwise convince a third party into going to war to help accomplish this. Also, if you are successful, you may be able to go even further in wiping them off the map.
- Win the game. Also a no-brainer. If you can win the game by another means while they are aggressively pursuing a military victory, then you win, and that's that.
- Combine the above two for a quick, cheap victory. Hold out against your own capital being taken, wait for them to eliminate all other players' original capitals, then declare war, and throw everything you have at theirs. If you're successful, you can drop the aggressive civilization's capital quickly, and win by Domination, potentially without ever engaging in any other war. This can be useful (it means every bit of military strength you can muster is put into just one, concentrated conflict), but if it fails, you've probably just lost your own entire military, while actively being at war with the only other civilization to have their original capital.
- As a more long-term solution, simply turtle up, and don't lose your original capital. A strong, concentrated defensive force with properly placed Medic promotions, and support from the city itself can stymie the AI's assault, and if they can't have your capital, they can't win by Domination.
- Befriend the aggressive Civilization. The effectiveness of this depends on the enemy leader, since some of them are rather prone to backstabbing, but many leaders won't attack you until very late in the game if you can successfully become a close ally. Additionally, this may set you up for a full-blown alliance. They're rare, but such an alliance would make their military victory into your military victory. Gods And Kings can help here through religion, since military civilizations often don't pursue their own religion, making it easy to spread yours to their cities, which tends to make them more apt to be friendly towards you.
There are also less direct routes:
- Whenever they go to war with someone, offer help. This could include units, cash, strategic resources, joining the war to make it be on two fronts, or using diplomacy to pull in other third parties.
- Try to cut off their economy and/or trade. Denouncements can help make other civilizations you've befriended be less likely to trade with them. In Brave New World there are options in the World Congress to embargo them, and also options to ban luxury goods they may have several of, limiting their happiness and trade options. You can also do your best to overtake them as ally of city-states they are allied with, cutting off another supply of cash/food/resources/etc.
- Espionage. Civ V may not allow spies to engage in direct sabotage like IV, but you can still put a spy into his capital to gain intelligence. Any time the aggressive civilization plans an attack on another, you may be able to learn of it through your spy, and warn them, giving them time to muster a better defense. In some cases, the aggressive civilization may delay, or not even ultimately declare war after all, if the defender is properly prepared for them in time.
- Harrassment. You may not be able to take the aggressive civilization's capital, nor outright defeat them, but if you can take a couple of their smaller cities, maybe pillage around their larger ones, you can still have an impact on their economy, and thus their military production. Ultimately, you don't need to prevent them from winning forever, just long enough that you yourself can win.