go to war (over someone or something)
To engage in an intense conflict with someone or something for some reason. Despite the connotations of "war," the phrase is often applied to commonplace situations. OK, so your assistant ordered the wrong pens. Do you really want to go to war over it? You stole the captain of the football team's girlfriend, and you're surprised he's ready to go to war?
go to war (over someone or something)
to wage a war over someone or something. (Often an exaggeration.) We aren't going to go to war over this, are we? Do you want to go to war over Sarah? Is she that important to you?