Why the Vietnam War was scarier than World War II

According to documents from the US Department of Defense, November 1, 1955 was the date when the US officially intervened in Vietnam and during that time, the US military support expert group in Vietnam was established.
According to Vietnam, the war between the two countries had already begun in 1947 when the US began to aid France so that France could continue to fight in Vietnam. However, regardless of the time frame, 1947, 1955 or 1965, at the beginning of the war, the US entered the battlefield with the mindset of a victor.
In fact, the American confidence was not without foundation, because it came from a country that had just won World War II, with a constantly expanding defense industry at home. Therefore, the Americans were more confident that nothing could stop them in the Vietnam battlefield. However, very quickly, they would realize that they were bogged down in a war that was more devastating than World War II and beyond the capacity of the entire American economy.