-1865: CIVIL WAR
When the compromises fell apart in the late 1850s, the South struck with the Dred Scott decision, essentially allowing slavery anywhere in the United States. The North had been reading Uncle Tom's Cabin and for the first time had a pretty good picture of just how barbaric Southern slavery could be.
Plus, the North was worried that the South might just march north with a factory's worth of slaves and take the North's paid jobs away. The South was worried that John Brown's botched raid on Harpers Ferry was the beginning of a North-sponsored slave rebellion. Neither of these situations would have happened, but a little paranoia about the other side is a traditional prelude to wars.
When Lincoln was elected, he had to sneak into Washington to become president. The train bringing him from his home in Illinois went through Maryland, a hotbed of Southern sympathy. The fear that someone would assassinate him before he ever took the oath of office was real.
The Southern states seceded and took over most of the federal forts in the South, but Lincoln wouldn't give up Fort Sumter, so the South started shooting. Lincoln was in a tight spot: Southern troops were nearing Washington, and all he had to defend the capital were a few ceremonial units including the Army band. Lincoln paced the top of Washington's hastily erected fortifications hoping to see reinforcements coming from the north. He even went to the Library of Congress and took out books on how to fight a war.
When the North got an army together, it didn't do them much good at first. The South won most of the early battles, fighting on its own territory with good generals. The North barely managed to turn back two Southern invasions, first at the Battle of Antietam and then at the Battle of Gettysburg. That was just enough.
Even though the North was fighting mostly for principle (one of the only times in history when an army has fought to free somebody else) while the South was fighting for its own land (usually worth three to one in fighting morale) and slave property, the North's determination through four long years of war won the day.