Annoying players who foil your plans

So I'm in a 6x speed map as France, and I've taken over Germany, Netherlands, Belgium, Lux, and Tunisia (yes tunisia) And the other actives are Russian empire (who has comm. russia, karelia & baltics) ukraine (who has romania, cossacks & bulgaria) and spain (portugal & switzerland) Yugo and Italy are active but aren't expanding. So I'm getting ready to attack Poland when czech declares war I attack quickly but i find ukraine is also attacking. So I figured that he would only take south czech but no, i see 15 ukrainian units going for Prague. But my RR and rocket artilleries are already bombarding prague and czech still have like 6 units there so either i stop bombarding or move to Prague and risk combatting the ukrainian troops. I retreat cuz russian empire is his ally and i dont want to be double teamed. But that wasn't a cool move on his part so I was going to help him attack yugo but now im not. I just don't like these kinds of players who ruin your plans for an empire.