A government is responsible for itself and it's people. It is not responsible for the well being of the world at large.

You're missing all the treaties the U.S. has ratified that do impose obligations to the rest of the world on it. But even if you ignore all of that, a state that has no qualms about mass murder outside of its borders is a dangerous, violent state that should be destroyed.

This is also a silly response to "what would you have done?". It's not about what states historically have done, it's about what the U.S. could have done that would have made it worth supporting. In the immediate aftermath of WWII the U.S. had military, political, economic, and social influence unparalleled before or since. It could have actually remade the world order, or at least tried, but it instead chose to continue imperialism with itself in the driver's seat. It was in no way forced to do this, and its decision is worth criticism.

US citizens generally approved of their government's actions after the war

U.S. citizens generally approved of the genocide of indigenous Americans, too. Just like democracy does not extend to voting to kill someone, it does not extend to committing genocide (which the U.S. supported and directly aided throughout the Cold War) and other war crimes, no matter how popular they are.

Just ask Japan - the US could have annexed the entire country and enslaved everyone.

"I could have killed my wife, but I just broke her arm! She should be thankful."

Instead, they denazified it

Hirohito remained emperor until his death and the imperial reign continues. Many war criminals were put into positions of power after the war. A couple examples include Yasuhiro Nakasone, the prime minister of Japan from 1982-87, who was directly involved in creating the "comfort women" system of sexual slavery during the war, and Nobusuke Kishi, the prime minister of Japan from 1957-1960. Here's a couple choice paragraphs from his Wikipedia page:

Known for his exploitative rule of the Japanese puppet state of Manchukuo in Northeast China in the 1930s, Kishi was nicknamed the "Monster of the Shōwa era" (昭和の妖怪; Shōwa no yōkai).[1] Kishi later served in the wartime cabinet of Prime Minister Hideki Tōjō as Minister of Commerce and Vice Minister of Munitions,[2] and co-signed the declaration of war against the United States on December 7, 1941.

After World War II, Kishi was imprisoned for three years as a suspected Class A war criminal. However, the U.S. government did not charge, try, or convict him, and eventually released him as they considered Kishi to be the best man to lead a post-war Japan in a pro-American direction. With U.S. support, he went on to consolidate the Japanese conservative camp against perceived threats from the Japan Socialist Party in the 1950s. Kishi was instrumental in the formation of the powerful Liberal Democratic Party (LDP) through a merger of smaller conservative parties in 1955, and thus is credited with being a key player in the initiation of the "1955 System", the extended period during which the LDP was the overwhelmingly dominant political party in Japan.

Japan continues to be a far-right haven to this day. Shinzo Abe, the recently assassinated former prime minister, was a direct descendant of Kishi and denied many of the crimes against humanity Japan committed during the war. He posed in a plane with the same numbers as the infamous Unit 731, a torture camp located in occupied China that was once under the control of his great-grandfather.

This only begins to scratch the surface of the far-right in Japan directly enabled by the United States. I hope you can see what a foolish statement you have made here.

Common among redditors, maybe