As someone who was born in Baltimore, and lived my entire life in the state of Maryland, I've come to know a good bit about the US national anthem, considering it was written in Baltimore. It was September, 1814, the late days of the War of 1812. British forces landed outside of Baltimore, and began to march toward the city, but they were stopped by US forces on the 12th, which is now a state holiday in Maryland. The British navy continued the advance by sea, and began to exchange blows with Fort McHenry the next day. The battle continued through the night. Francis Scott Key was in Baltimore at the time and woke up the next morning with no way of knowing who won the battle until he looked at the fort and saw the US flag flying over it, which obviously wouldn't be there if the British had captured the fort. That's when he wrote the lyrics to the song.
Also, it's become a tradition at sports games in most parts of Maryland, particularly at Baltimore Orioles games, which is where it started, to should "oooooh" at the line "Oh say does that star-spangled banner yet wave..."