1. The two atomic bombs in 1945. The war could easily have been won without them, although it would have taken a few more weeks, perhaps 2-3 more months.
2. The escalation of the Vietnam war. This actually caused 100,000s if not millions of lives. Totally irresponsible, particularily looking back at it today knowing that US eventually lost the war so badly.
3. Watergate. The worst political scandal in US history, but at least noone was killed or hurt.
Yeah, no. For your 1. at the top, it would have taken another year at the least and hundreds of thousands more lives and potentially a Soviet/US partitioning of Japan. Research Operation Downfall. Also, every Purple Heart plus the ones in storage awarded today are reserve stock from pre-Hiroshima and Nagasaki in expectation of Downfall.
Vietnam for my answer followed by War of 1812 and Reconstruction failures.
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