One of the most enduring American legends is that anybody, no matter how humble the circumstances, can become president. There are a ton of caveats to that, and the fact is most presidents (including the Founding Fathers) tend to be wealthy lawyers, but occasionally a pretty normal guy sneaks by and gets the top spot.
Harry S. Truman was one of those rags-to-commander-in-chief stories.
How humble, exactly? Dude was so humble he didn't even have a full middle name.
The "S" in Harry S. Truman stands for…S. He was named after his uncle Harrison (though they went with Harry) and couldn't decide on a middle name and so went with the letter S out of respect to both grandfathers who had S-names (Solomon Young, and Anderson Shipp Truman).
Harry started his life on a farm, and ended up holding a string of odd jobs that sound more like the resume of the weird guy who runs the public pool than the leader of the free world. He's the last president in history not to have a college degree (so far, anyway). He was, however, a veteran.
When World War I broke out, Truman wasn't going to sit on his hands while there was fighting to be done. He signed up, despite being a rather old thirty-three at the time. He finished the war as a captain in his artillery unit. (Source)
Truman returned and ended up getting elected as a district judge even though he only attended law school for a year. But this was actually an administrative position, so it's okay. The important part is that it was a stepping-stone to the Senate.
In 1934, he was elected to the Senate as a Democrat. He supported FDR's New Deal programs, which he was able to do directly from his position on the Senate Appropriations Committee. Hint: that's the committee that hands out cash. (Source)
In his second term, he served as chairman of the Senate Special Committee to Investigate the National Defense Program. Quite a mouthful. Basically, Truman was looking for wasteful spending there, and this—a Democrat being a watchdog for wasteful government spending—helped earn him a reputation for integrity. (Source)
In 1944, Truman got the nod to run for Vice-President. Unfortunately for him, and arguably the world at large, FDR died eighty-two days after he was reelected for a record fourth term. Truman was now Prez.
Truman had zero experience in foreign policy, and all of a sudden he was in charge of the end of World War II, escalating tensions with the Soviet Union, the spread of communism, and oh yeah, this little nuclear program they had going out in the desert. Truman made the decision to drop two atomic bombs on Japan, which effectively ended the war for them.
Truman outlined a twenty-one point plan of action for domestic policy, which became known as the Fair Deal. It was a logical outgrowth of FDR's New Deal program, and emphasized raising the standard of living for as many Americans as possible.
As for foreign policy, he tapped his Secretary of State, George C. Marshall for that, who came up with a little thing called…the Marshall Plan. Truman also stemmed the tide by organizing the Berlin Airlift, getting crucial supplies to a surrounded and blockaded West Berlin, as well as founding NATO.
Yeah. That's a lot for a guy whose beginnings were so drab his parents didn't even give him a full middle name.
Truman narrowly won reelection in 1948 over Thomas Dewey. (There's a famous photo of him holding up a newspaper with the headline "DEWEY DEFEATS TRUMAN," making him look like the only parallel-universe president in history.)
Truman's most controversial foreign policy decision was the Korean War, so called because it broke out when communists from the north invaded the south. Truman committed U.S. forces without a formal declaration of war from Congress. This is important as it's technically illegal (and has been done several times since, as Korea was now a precedent). The end result was that Korea was split into two different countries, which continues to this day.
Despite the fact that Truman could technically have run for another term, he chose not to, and retired to his hometown of Independence. Truman chose to take it easy, and apparently this was good for him—he lived another twenty years playing shuffleboard, eating dinner ridiculously early, and doing all those other cute retiree activities. (Source)
There's no way he saw any of this coming when he was working in that mailroom.