We had a very relevant experience here:
http://webdiplomacy.net/board.php?
gameID=161305---
Three teamed up to stop a solo, then Turkey withdrew specifically to thin the draw. Given the circumstances, a reasonable approach, and I think leads to an I nteresting question:
If there is no danger of a solo, do you thin the draw on principle? Or protect your former ally out of a sense of loyalty?
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Probably quite varied game-to-game. Even in this case, which was a league game, it was a difficult decision to make; however, on the balance I would say that the following priorities can be pretty consistently implemented regardless of game specifics:
1. Win
2. If you can't win, draw with as few people as possible
2a. If there is a choice between drawing with more and trying for fewer but risking a win from a prepared opponent, draw with more.