Lincoln was fighting a war because a war was forced upon him. But you've been told that before.
Lincoln was fighting a war because that is what he fervently wanted to do. Any time after taking office he could have ordered the evacuation of the property in dispute. It no longer served any purpose for the Union anymore anyways.
He didn't want to defuse the crises, he wanted to build it up thinking his superior resources would make quick work of the defiance of his authority.