Non-working alliances

The alliance system in SR isn't set in stone. In other words, an Ally will (sometimes) come to your aid, other times not. They take a diplomatic penalty for violating the treaty. The big thing is casus belli (CB) against the attacking country. If you are playing USSR and have only attacked West Germany, then you'll fight NATO units in W. Germany. Once it falls, they'll leave. Odds are that you'll get a declaration of war soon from another NATO member, as bad relations if left alone will just build up. Keep in mind, if you took out W. Germany, and even if you were fighting other NATO units (like Belgium) that doesn't mean you get to invade Belgium--you have to DoW them, which in turn makes it a lot more likely for others to DoW you. At a certain point, if you attack enough countries, even your allies will break treaties with you.