When players' units are destroyed on AI or neutral planets, the AI collects a portion of their metal cost as salvage. As with human salvage, this amount decreases slowly over time.
Once the amount of metal collected passes a certain threshold, a message saying "AI X Reprisal Warning Level Y" (where X is the identifier of an AI and Y is a number) appears in the notification area. After a few minutes, this warning will disappear and a special wave that includes "(Reprisal)" in its info will launch against one of the players' planets.
If players loses enough units quickly enough the "AI Reprisal Warning Level" will increase.
A reprisal wave triggered by a level 1 warning is 1.5 times the size of a regular wave. That size ratio is doubled for each successive warning level.
- Level 1 is 1.5x
- Level 2 is 3x
- Level 3 is 6x
- Level 4 is 12x
- And so on...
The amount of metal required to generate reprisal waves increases with AIP, but this is balanced by the fact that the total metal value of the player's fleet increases as well. In general, a total fleet wipe will cause a level 3 or so reprisal.