Following is condensed from an article by Isaac Asimov in the March 1991 issue of The Magazine of Fantasy and Science Fiction
In 1812, the United States went to war with Great Britain, primarily over freedom of the seas. We were in the right and, for two years, we held off the British, even though we were still a rather weak country.
Great Britain was in a life-and-death struggle with Napoleon. In fact, just as the United States declared war, Napoleon marched off to invade Russia. If he won, as everyone expected, he would control Europe, and Great Britain would be isolated. It was no time for her to be involved in an American war.
At first, our seaman proved better than the British. After we won a battle on Lake Erie in 1813, the American commander, Admiral Oliver Hazard Perry sent the message "We have met the enemy and they are ours."
However, the weight of the British navy eventually beat down our ships. New England, hard-hit by a tightening blockade, threatened secession.
Meanwhile, Napoleon was beaten in Russia and in 1814 was forced to abdicate. Great Britain now turned its full attention to the United States, launching a three-pronged attack. The northern prong was to come down Lake Champlain toward New York and seize parts of New England. The southern prong was to go up the Mississippi, take New Orleans, and paralyze the west. The central prong was to head for the Mid-Atlantic States, and then attack Baltimore, the greatest port south of New York.
If Baltimore was taken, the nation, which still hugged the Atlantic coast, could be split in two. Therefore, the fate of the United States rested to a large extent on the success or failure of the central prong.