War

§ War happens to everybody. Sooner or later even the most peace-loving leader is going to find himself fighting one of his neighbors - perhaps because all of the neutral lands have been taken, perhaps because his opponent controls resources he covets, or perhaps merely because he looked at him funny. You do not need to conquer the world to Win a game of Civilization IV, but you do need to be able to defend yourself against other civilizations until you do win.
§ 

How Wars Begin

§ There are a number of ways that a civilization may initiate war against another:
§ Declare War
§ You can declare war on another civilization while conducting Diplomacy with that civilization's leader.
§ Attack a Unit
§ When your unit enters a space containing a foreign unit, the program will ask if you want to attack that unit or enter peacefully. If you choose the former option, your unit and the other unit will battle, and your civs will be at war. If you choose the latter option, your unit will enter the other unit's square peacefully, and no war will ensue.
§ Enter Another Civilization's Territory without Signing Open Borders
§ When you direct a unit to enter another civilization's territory, the program will remind you that doing so will cause war with the other civilization. If you tell the unit to go ahead, the unit moves and you're at war.
§ Exceptions: There are a couple of exceptions to this rule. If you have an Open Borders agreement with the civilization, then no war ensues. Caravels and some "invisible" Units do not cause war when entering another civilization's territory.
§ Have a Defensive Pact with a Civilization that is Attacked
§ If you have a Defensive Pact with another civilization and that civilization is attacked, you are automatically at war with the attacker. (Note that if your ally declares war on another civilization, your defensive alliance is revoked; you are not automatically at war with your ex-ally's new foe.)
§ Have a Permanent Alliance with a Civilization that is Attacked or Attacks another Civilization
§ If you have a Permanent Alliance with another civilization, you are automatically at war with any civilization at war with your partner, no matter who started it.
§ 

Effects of War

§ Units Expelled from Enemy Territory
§ If you had any visible units in your new enemy's territory when war was declared, all such units are expelled to your territory or neutral territory. The same happens to any of your enemy's units within your borders. (Note: This stops the shabby trick of making an Open Borders agreement with somebody, moving a lot of troops next to that civilization's capital, and then declaring war and assaulting the target city... Shame on you for even thinking of it!)
§ Trade Agreements
§ If you have any trade agreements with your new enemy, all such agreements are now void. If your enemy's territory sits aside a trade network with a third civilization, that trade network is cut and you can no longer trade resources with the third civilization until a path is opened.
§ Movement Through Enemy Territory Slowed
§ If you have an Open Borders agreement with a civilization, your units can use that civilization's roads. If you are at war with another civilization, however, you cannot use the roads in that civilization's borders.
§ War Weariness
§ Your cities may become afflicted with War Weariness, which causes their population to become unhappy.
§ Combat
§ Combat will occur any time that one of your units tries to enter an enemy unit's square, and vice versa. See Combat for details.
§ 

Achieving Peace

§ War ends when one civilization loses all of its cities and is thus out of the game (see Victory). Short of total destruction, wars may be ended Diplomatically when both sides agree to a Cease Fire or Peace Treaty.
§