Why didn't the North let the South secede?

[SnohoDem]

Well, the title asks the question. Here's why I thought of it. Recently we're seeing the secession petitions, and 'Lincoln' is on at the movies. I won't see 'Lincoln' until it's out on DVD, but tonight I watched the miniseries based on Gore Vidal's book.

There's a heck of a lot of smart, well educated people here, so I can't think of a better place to ask this question.

Why didn't the North just let the South go? Although slavery was the central issue in the Civil War, my limited understanding is that the war was fought to prevent the slave states from leaving the Union. Why? Wasn't the North's position that slavery would not be allowed in border states or new states, but that it would still exist in the deep South? Even the Gettysburg Address says nothing about slavery except, "that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth."

Was the political pressure from the abolitionists so great that Lincoln felt he had to prosecute the war?

Thanks for any answers.