The British, as a last ditch effort after failing to take Baltimore by land and unable to enter the harbor due to Fort McHenry's cannons, deliberately sunken ships, and low water levels, decided to withdraw to long range bombardment.

His knowledge of what was happening inside the Fort was somewhat limited. He witnessed the bombardment from a ship outside the harbor and wrote the poem on board the ship. The events of the Start Spangled Banner are a bit of a misconception by Francis Scott Key. Their blood has wash'd out their foul foot-steps' pollution, No refuge could save the hireling and slave, From the terror of flight or the gloom of the grave

The third verse is unusual because it contains anger directed at former slaves who fought for the British and helped defeat American forces at The Battle of Bladensburg: The lyrics of the 'Star Spangled Banner' come from Francis Scott Key's poem 'Defense of Fort McHenry' which he wrote on-board a negotiation ship during the Battle of Baltimore, War of 1812.

Origin of the star spangled banner song