He was a lawyer and amateur poet who witnessed the bombardment of Fort McHenry by the British during the Battle of Baltimore. Key, who had been trying to secure the release of a prisoner held by the British, was held on a British ship during the attack.
He watched throughout the night, anxiously awaiting the outcome of the battle. As dawn broke, he saw the American flag still flying over the fort. Inspired by the sight and the victory it signified, Key wrote the poem, which captured the emotions and pride of the nation during a time of great conflict.
So, Key wrote the poem as a reflection of his experience and patriotic sentiment during the Battle of Baltimore.