inhereting a war

So i need some help here because i cant find the info on the wiki and i dont really dare to gamle on this war due to im most likely going to be dragged in to a really annoying war if done wrong.

im atm with war with Brandenburg and taken al but one province that lubeck was a bastard son and nagged before i could get there. so now im thinking of just vasselising brandenburg anyway inherit the war and just steal some land back from lubeck. Problem is he have some annoying alliances and im not sure how this will work. Will it be seen as a defensive war for me so i can call in my allies or is it me alone vs him and al his allies? or is it just him and me?

Thanks in advance for information regarding the issue.