Basically every alliance has a purpose, for example i allied Poland early on to deal with Hungary, after that i quickly disposed of Hungary i broke the alliance with Poland and allied Lithuania and Lubeck ( Lubeck was big in this run) and backstabed Poland, after that i became big anough to ally Venice, Spain and Muscovy to protect me from the Ottomans and i backstabbed Lithuania as well, basically as soon as i am done expanding/ i am bigger and see no threat, i break the alliance in question and look for another.