War with parent civilisation

Exterminating your own civ will not help with your migrant problem

When your civilization gets exterminated, either through embarking as a civ with very few (less than about 7) members, or through the actions of either you or other beings in the world, migration, with the exception of the first two free waves, stops.

Thus "winning" your civil war will mean you will permanently stop all migration to your fort. It does mean that caravans may arrive once again.

Causes

You can start a civil war by souring relations with your parent civ. For example, these actions may cause war:

• Attempting to raid a location belonging to your own civ
• Killing a diplomat
• Killing the outpost liaison
• Stealing from, or Killing your own traders
• Denying requests for artifacts

Other actors may also cause a civil war in a multitude of complicated interactions within the world's history, some of which may actually be intended. For example, in this bug report, a dwarf simultaneously acted as a representative of an Elven government and a member of their own civ (probably trying to get his own site to stop cutting trees or some such thing), and caused a civil war. Reading it, here's some ways this may happen:

• By accident

Dwarven site A attacks enemy site X, and succesfully conquers it. Some time later, dwarven site B, not knowing of the attack, launches their own attempt. Mistakenly they end up raiding and fighting their own civ. This causes chaos to ensue with all the sites declaring war on eachother.

• Multiple allegiances

Treasonous actions of high-ranking individuals with multiple allegiances may lead to civil war, with some portion of a civilization backing them, and another portion not. E.g. a Fortress mayor that is member of an elven civ getting into conflict with another fortress. This can be due to allegiances to multiple civilizations, or relations with individuals from other civilizations.

• Royals and Nobles

Currently, I do not believe some kind of political intrigue like a succession crisis or a rebellious noble is a possible cause right now in the game, but it could be a likely cause for civilizations to end up in civil war, which may lead to a split, later on in the game's development.

Fixes

You can maybe end the war the regular way

Wait for a diplomat to arrive. They may offer you a peace treaty. Accept it and it should end the conflict.

Use DFhack to modify the game

The state of war is kept as a 'relation value' which exists for every pair of civilizations within a world. You can directly modify this value in memory using DFhack. Also see this forum topic for more info.

Sources

1. The wiki article on War.
2. A useful Bug report on Civil war