Europa Universalis IV

This is another achievement you just get by playing the game. Simply get one or more ally and sooner or later those allies will start a war or be attacked. Accept the call to war prompt and win one of those wars. and this achievement is yours.

However these kind of wars is very time consuming and all you usually get out of them is prestige, usually the main participant claim all the land and gold. They have their use if you do your own peace deals and lacks reason to go to war.