The fort was under union control before it was attacked by the south (South Carolina) and began the civil war.
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The South seceded from the Union. In their view, this made Fort Sumter part of their territory, being held by Union forces. They demanded that the Union soldiers surrender the fort. Lincoln had to either order the men holding the fort off... or he had to send a re-supply ship to provide the men at the Fort with the means to stay. He ordered the Fort to be re-supplied. This was taking to position that the Union was still the owner of the territory. South Carolina took this to be a provocation because it denied their rights to their own territory. If they were no longer part of the Union, then the Union could not have forces on their land. They began shelling to make this point.
South Carolina's assault on Fort Sumter
Lincoln, by openly deciding to resupply the fort, provoked the South into a rash attack. After the fort fell to Southern attack, public opinion widely favored war with the Confederacy. On April 15, three days after the surrender of the fort, Lincoln called on Northern states to provide troops to "protect the Union." While many felt that Lincoln had made a miscalculation, others saw his actions as deliberately designed to cause the eruption of hostilities, with a Northern takeover of the South being the only solution to his earlier analogy of the US as "a house divided" by the institution of slavery.
When the Battle of Fort Sumter began at 4:30 a.m. on April 12, 1861, no reports of bad weather were made. It is believed to have been above 62 degrees and clear, as most fighting would never have been initiated if the weather was inclement.
Well, honey, the Confederates attacked Fort Sumter because they wanted to assert their authority and control over the secessionist state of South Carolina. It was basically their way of saying, "We ain't playin' around, y'all." So, they fired the first shots of the Civil War and the rest is history.