It'll be over by Christmas

I thought now was a good point to recap when Diplomacy ends, as we've moved past the end of the beginning and are firmly into midgame to endgame territory.

There are two endings, a win or a draw.

Win

In order for a single player to win outright they must sucessfully control 18 supply centres, (there are 34 in the game and so this represents one over the halfway point, or an overall majority).

This can only be a solo victory, you get nothing for a coalition, not in this game.

Draw

If all players can agree a ceasefire then the game finishes in a draw between everyone remaining in the game. A draw is entirely equal whether you have one unit or seventeen units left on the board. Players who have been eliminated don't get any of the glory of the draw.

There must be a complete concord amongst the remaining players in order for a draw to be legitimate. If one player, no matter how many units they have, does not agree to a draw then there cannot be one.

To declare the game a draw, I must recieve a message from every remaining player in the same turn saying that they agree to the draw.

There are also some articles on mid and endgame strategy on The Diplomacy Archive.