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Last night I was playing a two player game and my opponent swapped to a Diplomatic race partway through the game. they managed to surround me, effectively hemming me in and limiting my score. In the end I lost by two points.
What tactics can be used when facing an opponent with a diplomatic race? Are there any preparations I should make when playing knowing that a diplomatic race is in the available race selections, even if it's not yet selected?
As @bwarner commented, I'd say abandon all regions and start invading from the maps borders
From the rules (emphasis mine):
If a player wishes to free up some more Race tokens, he may opt to entirely empty up some - or all - Regions, leaving no Tokens there; but in this case, these now abandoned Regions will no longer be considered his, nor bring him any Victory coins. If the player chooses to abandon all the Regions he previously occupied, his next conquest must follow the same rules as its First Conquest (see First Conquest, p. 4).
It's a tough one. I'd say that it's pretty hard to be completely surrounded, so focussing on avoiding that is a good first step. If you see the diplomatic option come out, you could consider spreading to increase your race's surface area as much as possible, making it difficult or impossible to surround. Or take the diplomatic race first!
If the unfortunate happens, I'd say the best course of action is to immediately decline and next turn appear at the other side of the board.