In 1940, the United States was on the sidelines of World War II. While it was obvious that it would not be able to stay out of the war, the country had not yet been attacked and there were several people who wanted Roosevelt to stay out of "policing" the world. There were many who saw that Adolf Hitler would not be satisfied by just taking over Europe. He planned to continue his westward march. Into this uncertainty came Captain America, defender of the constitution and of the United States.
Originally named Super America, Cap's name was changed so that he could stand out from the crowd. From his very first cover, it was obvious that his creators felt strongly that the United States should enter World War II against Germany and Hitler.
After the war, Timely Comics (the precursor to Marvel Comics) found Cap's adventures to be a hard sell. He was discontinued in the 1950's, only to be revived in 1964 by the renamed Marvel Comics under the expert pen of the Generalissimo himself, Stan Lee. Cap has stayed in print ever since.
If you happen to find a mint condition Captain America number one hiding out in an attic or basement somewhere, you're in luck. At a recent auction, the title fetched $340,000.