“Having served in the trenches during WWI, Mr. Truman may have empathized with the troops who would have to fight their bloody way across the Japanese home islands.”

Nonsense!
“On August 29, 1918, Battery D fired 500 rounds of artillery at a German position. When the Germans returned fire, some of Truman’s men panicked and ran. “My greatest satisfaction is that my legs didn’t succeed in carrying me away, although they were very anxious to do it,” Harry wrote Bess later. Cursing and yelling, Truman drove his men back to their positions, and successfully repositioned two of his four guns. For the remainder of the war, Truman led Battery D across the French countryside, hammering German positions and never losing a man. On the battlefields of Europe, he experienced the success that had long eluded him.”

pinched from here;

http://www.pbs.org/wgbh/amex/truman/sfeature/sf_early.html

Truman was a battery commander (4 tubes). As a Captain, an over ranked one, IIRC, To say he served in the trenches is a bit of propaganda. Harry was always better at propaganda then anything else.
AS far as his courage, where? He DID NOT make the decision to drop the bomb. He could have made the decision to not drop the bomb but that would have required courage. The decision to drop the bomb was made while Truman was a Senator. Targeting was done while he was VP. FDR was too smart to let an incompetent coward like Truman anywhere near the machinery.
Truman was a weasel.