1732 - Samuel Augustus Adams is born.
1749 - 17-year-old Samuel becomes an apprentice typesetter.
1754 - 22-year-old Samuel volunteers for the French-Indian War.
1763 - 31-year-old Samuel returns home at the end of the French-Indian War.
1766 - 34-year-old Samuel gets married & starts his own press.
1771 - 39-year-old Samuel's wife is killed by British troops. He begins publishing separatist papers.
1775 - 43-year-old Samuel dies at Concord & Lexington. He is found by the wizard Shazam & given the chance to become his champion, but he rejects the wizard's power in the favor of the strength of his Nation. He becomes the elemental spirit of America, Minuteman.
1783 - Minuteman stops appearing when the Revolutionary War ends.
1812 - Minuteman begins to appear again during the War of 1812.
1815 - Minuteman stops appearing when the War of 1812 ends.
1846 - Samuel appears again during the Mexican-American War, this time as Brother Johnathan.
1848 - Brother Johnathan stops appearing when the Mexican-American War ends.
1861 - Samuel manifests as two separate beings during the American Civil War, Johnny Reb & Billy Yank.
1865 - Johnny Reb & Billy Yank stop appearing when the American Civil War ends.
1898 - Samuel manifests as Uncle Sam for the first time during the Spanish-American War, and is first used as a propaganda figure. He stops appearing when the war ends.
1914 - Uncle Sam manifest again during World War I, fighting overseas for the first time.
1918 - Uncle Sam stop appearing when World War I ends.
1940 - Uncle Sam manifests again during World War II, assembling the Freedom Fighters to fight overseas.
1945 - Uncle Sam stop appearing when World War II ends, marking the last time America has declared war, ending his appearances.
The realization that Uncle Sam is actually a DC Comics superhero always seems odd the first time you discover it. It shouldn't be that odd... he's a fictional character, and SOMEONE must own the rights to him. In his case, he was published by Quality Comics and then later acquired with the rest of their character line by DC. The Freedom Fighters were created as an alternate history team of heroes battling against a victorious Nazi regime, creating a home for many of the less popular Quality characters.
The idea that Sam is a spiritual entity representing the strength of American Patriotism was introduced to his story after his integration into the tapestry of the DC multiverse, as well as his earlier incarnations as Minuteman, Brother Johnathan, and the dual identities of Johnny Reb & Billy Yank. It gave him a sense of scope that was very clever.
In order to better integrate Uncle Sam and the Freedom Fighters into our unified timeline, we've put in a little work to expand Sam's story. Rather than being an entity crafted by the founding fathers using occult magic (which... what?), we've tied his magical origin to the wizard Shazam. The role as the Wizard's champion is already well established to be a very powerful one, so to have him chose to reject that role in favor of being the manifestation of the patriotic spirit of the nation actually has a certain symmetry that works immediately.
This was actually a fun history to make, because by designing the character so that he manifests whenever America is at war we got to do a little bit of history research and find the specific years when American was actually at war. In fact, this principal actually completely explains the fact that Sam hasn't appeared again since WW2, as American stopped declaring war at that point in favor of 'conflicts'. Korea, Vietnam, Iraq and so on aren't 'wars', strictly speaking. This idea works really well right now, although if America ever did actually declare war against someone again, the story sort of falls apart.
Although that would probably be the least of our problems.