Now, in a more conventional situation, let's say you have Vie, Buda and Rumania and he has Boh, Galicia and Ukraine. Then the worst thing you can do is to attack all 3 of his units cos he will win.
Why? cos as fasces said, the defending unit can't cut the support on it's own territory.
So Buda is attacking Galicia, but Galicia is moving somewhere, supported by another unit, then Buda would porbably end up in Galicia. Say Galicia is moving to Rumania, supported by Ukraine, then Rumania can't break Rumania's support, cos it cannot break support on its own territory.