For a huge number of smaller reasons that added up:

• He had big shoes to fill. His ascension after Roosevelt's death gave him high expectations from the start.

• Incessant conflicts with the Senate. Many of the bills he vetoed defended leftists and unions at a time of rising tensions with the USSR and the start of the Red Scare.

• The "loss of China". That phrase featured prominently in politics at the time. Today, most analysts agree with Truman in that the Chinese nationalist regime, in Chiang Kai Shek's own words, was "rotten to the core" and beyond saving. However, Truman's lack of action was panned by the Republicans and he was blamed for "losing 600 million people to Communism"

• The debacle of Korea. After American troops advanced to the Yalu river, the Chinese PLA in a stunning string of victories encircled and destroyed numerous UN divisions in a surprise attack. General MacArthur, seen at home as a war hero, was sacked for overextending his forces, clashing with the President, and defying a direct order. He was invited by the Senate to make a speech in defiance of Truman. It was the first time in history a General had given a "farewell speech" to Congress without the President's consent. The war dragged on from 1951 to the end of Truman's presidency in stalemate. Americans were used to "winning", according to one scholar, and this was the first war where there was no conclusive outcome. Ultimately, during the Cold War, the US would see many more of those, but Korea was the first, and not popular.

• Lack of presidential personality. Truman "had no charisma" and a distaste for manipulation. (Source: Man of the People, Hamby) Even though he got a lot done and was fast and decisive, his public persona was lacking. Commentators at the time usually remarked his speeches were just cluttered lists of facts. He wasn't an impressive political figure and didn't seem to live up to past presidents like Roosevelt and Coolidge at building a popular public persona.

For ordinary Americans, Truman looked like a weak, somewhat bumbling leader during a time of challenge against Communism. It's important to note the young superpower was still innocent: Americans' only experience with foreign involvement was a stunning, rapid victory against a weaker German Western front and a relatively bloodless and easy island hopping struggle against Japan, both coming with immediate moral validation in the form of the war crimes tribunals. They expected the same in the Korean conflict, and got something very different.