40d:War

War is a specific state the player is at with another civilization that may either be present at embark or be caused during fortress mode by certain incidents. A proper war will be signaled on the embark screen with "WAR" and will be denoted in-game on the civilization screen.

The player can be at war with any other race, but not with any dwarven civilization.

Starting out at war on embark can be achieved by checking the available dwarven civilizations and the tiles they occupy (blue) for symbols that normally signify settlements of other races. Then move the cursor into a region relatively close by, and start a new fortress. If a fort is started too far away, perhaps into the territory of another civilization of that race, 'war' will disappear.

The general state of hostility most goblins show towards the player, complete with ambushes and sieges, doesn't even count as war. It is signified instead by "--------" on the embark screen.

Causing war

Allowing envoys or traders to die on your map, whether intentionally or not, is considered a hostile action (even if they die of natural causes). Too many of these actions can cause the civilization to attack the player. It may only take a single diplomat's death to start a war. Not keeping to agreements, clearcutting forests (in the case of elves), seizing whole caravans repeatedly and annoying elven traders by offering them wood till they leave are other aggressive actions that may lead to war.

Elves

War on embark

If you start out at war with elves they won't send caravans and trade with you. Their civilization may (will?) also be missing from the Civ Screen until they ambush you. As always, elves will not siege, but ambush, much like goblins. An elven ambush will not be announced as such but can be recognized from the map zooming to the (invisible) troops' entrypoint. Once one of your units detects them, they turn visible and the "An ambush! Curse them!" message is announced.

Elven ambushes arrive about the same time the caravan would; middle of early spring.

The first ambushes may arrive as late as when your fort qualifies as barony.