Why do people take their hats off during the National Anthem?

The custom dates to the Civil War era when Union soldiers would bare their heads to honor their flag while it flew over battlefields. Although a salute would be more formal, Civil War veterans had just one hand between them, and they needed to keep that hand free to hold their rifles. By 1870, the War Department decreed that all service members should stand at attention and face the flag during the playing of the national anthem. That decree eventually became tradition for civilians, too.