Why was it so important for the north to hold fort sumter?

It was symbolic more than anything. Today the US has military outposts all over the world. But every one of those outposts is there by the consent of the government of the country where it is located. This is something never before seen in human history. And it was not that way in 1860.

The Confederates reasoned that they were a new nation. They had withdrawn from the United States, and set up a new nation; therefore the United States had no business maintaining its military forces in Confederate territory. They had been asked to leave, and did not. A sovereign nation does not allow another nation to keep its military forces within its borders, unless it has agreed to allow it. Otherwise its an act of war.

After the US had won its independence from Great Britain in the Revolution, the British held on to Detroit for twenty years, with soldiers in Fort Detroit, stirring up the Indians. Everyone remembered this in 1860. In the 1790s the US had been powerless to force the British to get out, as they had agreed to do in the peace conference at the end of the Revolution. It had been a real sore point in relations between the US and Britain.

There were three or four forts around the Confederacy like this

• old masonry forts built for coastal defense by the US government.

There was one at Pensacola, Florida, and another at the tip of Old Point Comfort, in the Chesapeake Bay of Virginia, Fortress Monroe, which is still an active US Army base today. The Union held on to these forts as well, and they were never captured by the rebels. Others of that old system of forts were taken by the rebels in various places.

Fort Sumter was actually unfinished when the Civil War began. The Union troops were stationed at another fort on the mainland in Charleston Harbor. The South Carolina authorities were trying to get them to leave, when one night they sneaked out by boat to Fort Sumter and stayed there.

After Fort Sumter surrendered the Confederates allowed the Union troops to hold a ceremony and haul down their flag, and fire a cannon in salute to it. The cannon they fired blew up and killed one soldier, who was the only Union soldier to die in the whole episode. Then the Union troops were put on a ship and sent off to New York. So it is kind of remarkable that an almost completely bloodless incident stirred up such a storm. Both sides thought the war would be easily won, and looked forward to beating up on the other guys. Four years later they were very much sadder and maybe even wiser.