Final answer:
The poem 'The Star-Spangled Banner,' which became the U.S. national anthem, was written by Francis Scott Key after he witnessed the British bombardment of Fort McHenry in 1814.
Step-by-step explanation:
The poem that later became the national anthem of the United States, "The Star-Spangled Banner", was written by Francis Scott Key in 1814. Key was inspired to write the poem after witnessing the unsuccessful bombardment of Fort McHenry in Baltimore Harbor by the British navy.
The battle was part of the larger conflict known as the War of 1812, which also included the burning of Washington, D.C. Despite the intense British attacks, the defenders of Fort McHenry held steadfast, and upon observing the American flag still waving after the bombardment, Key penned the verses that would eventually be adapted into America's national anthem.