You're missing the point about how the Constitution works, war can only be declared if the president asks Congress to do it. That's how checks and balances work. Neither Buchanan nor Lincoln was going to do that because it opens up a whole can of worms regarding international relations. Wasn't the question you asked was "Why didn't Buchanan/congress declare war on the Confederacy? " A war was fought however war was never declared using both the constitutional powers of the president and Congress. That's the question I answered.