Well, we really need to find a way to disable diplomacy in PBEM games. Usually, I enable the diplomatic options myself in the game settings just to counter my opponent if he uses diplomacy in our game. Except that I can't do anything if a single group of attempts manages to tip a country into war without me being able to intervene. Example in my last game:
I understand better why my opponent didn't put troops on the Yugoslavian border. Not that Yugoslavia is very powerful, but he will now be able to fully mass his troops on the Russian border for Barbarossa. Moreover, the Yugoslav troops will be perfect for managing the security of the railways against the partisans.
It's true that I've experienced worse: in one of my games where I played as the Ally, my opponent tipped Yugoslavia into the Axis camp on the first turn!!! Or how to lose the game without even having clicked on a single unit.
I have nothing against Spain or Turkey joining the Axis Power before the US enters the war, under specific conditions (invasion of England, capture of Baku). But this is still very painful.
Once again, I suspect it's too late for Warplan. But I hope Alvaro will consider it for Warplan 2.