This article gives the basic reason why the Battle of Fort McHenry (where the "Star-Spangled Banner was written), determined the fate of the nation. By September, 1814, the British had already captured and burned Washington, D.C. But they had not won the war, and could not win it, unless they could shut down the docks and shipyards in Baltimore, from which the Baltimore Clippers sailed.
If the Fort had been "reduced" and surrendered, the City would have fallen. If the City fell, the shipyards would be burned. And if the Baltimore Clippers had been eliminated, the British would have won the War of 1812, and returned the American states to the status of colonies of the Crown.
And now you know "the rest of the story."
Congressman Billybob