Thanks to Omnicomic.com for remembering Uncle Sam in time for the Fourth of July!
According to Wikipedia, Uncle Sam dates back to The War of 1812. The image of Uncle Sam was actually created in 1852 and as such, might really be the first superhero ever. That might be a stretch, but Uncle Sam, the comic creation, is more than 60 years old and was created in 1940 by Will Eisner (yes, really)! Uncle Sam, the embodiment of the American Spirit, has always reappeared at times the USA was under threat.
Sorry Brits, but Uncle Sam predates Lord Kitchener Wants You, and patriotic posters from many other nations.