What event inspired it?

The event that inspired The Star-Spangled Banner was the British naval bombardment of Fort McHenry in Baltimore Harbor on the night of September 13 to 14, 1814 during the War of 1812. Fort McHenry was a star-shaped fortification on Locust Point that protected Baltimore Harbor. Three weeks earlier, on August 24, 1814, British forces under Major General Robert Ross had defeated American militia at the Battle of Bladensburg and burned the U.S. Capitol, the White House, and other federal buildings in Washington, D.C. The British then turned to Baltimore, the country's third-largest city and a center of privateering against British shipping.

Beginning on the evening of September 13, 1814, Vice Admiral Alexander Cochrane sent a fleet of 19 British warships, including five bomb vessels (HMS Terror, HMS Volcano, HMS Aetna, HMS Devastation, and HMS Meteor) and the rocket vessel HMS Erebus, into the Patapsco River and pounded the fort with mortar shells and Congreve rockets for about 25 hours. The fort was held by about 1,000 American soldiers, sailors, and militia under the command of Major George Armistead. The bombardment was loud, dramatic, and destructive (the British fired between 1,500 and 1,800 shells and rockets), but Fort McHenry's walls held. American casualties were remarkably light: four killed and 24 wounded.

Francis Scott Key, a 35-year-old Maryland lawyer, was aboard a British truce ship in the harbor, having gone out to negotiate the release of Dr. William Beanes, and was detained for the duration of the attack. From the deck of the ship he watched the bombardment through the night. At dawn on September 14, 1814 he saw the fort's enormous 30-by-42-foot garrison flag, which Major Armistead had ordered raised in place of the smaller storm flag that had flown during the attack, still waving over the ramparts, signaling that the fort and the city had held. The British abandoned the assault later that day, withdrew from the Chesapeake, and ended their invasion of the mid-Atlantic.

Key's poem describing the all-night bombardment and the morning sight of the flag became the lyrics of The Star-Spangled Banner. The Treaty of Ghent, signed on December 24, 1814, formally ended the War of 1812.