I think that's a rather poor way to look at things; you shouldn't have strategic absolutes. It's not unheard of, for instance, for France to cede brest to England if E/G forms at the start, in return for England betraying Germany.
The real threat to England until very late in the game is the spaces that produce fleets - Brest, Kiel, Berlin, and St. pete., and maybe Mar to a lesser extent. With Brest taken, France is no longer a serious threat, and Germany is much more likely to be the one to invade England one day, not France.
The only alternative in that situation is to turtle and pray that Russia becomes strong enough to give you relief and that Italy doesn't decide to dogpile and try to take centers while you're weak. If you're reasonably confident of those two things happening, then by all means fight tooth and nail until you get relief.
But if, say, Turkey has gutted Russia and he's struggling just to defend himself, or if Italy has decided to move westward, staying enemies with both Germany and England is a suicide wish. Ceding brest can cause England to be your ally, the most proximate threat then being Germany, and also Germany seeing all the gains from the attack go to England and not himself may also decide to abandon the alliance.
Similarly, if I were Russia under attack from A/T, defending st pete would be a low priority for me. If losing it to an English fleet was necessary to ensure ARMIES won't pour in from the west or north, I would do it.
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Mod Policies
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