Why didn't the participants of the Korean War also fight in the Vietnam War?

Question

In the Korean war from 1950 to 1953, a lot of countries including the U.S., the U.K., France, Canada, Australia, and even Colombia, the Philippines and Ethiopia participated in the war under the U.N. resolution. However, those countries didn't participate in the Vietnam War. The two wars seem to have similar characteristics, i.e. a war caused by (fought for) an ideology, a proxy war between the U.S. and the former USSR.

1. Was there any particular reason that the U.S. couldn't get necessary supports from its allies such as the U.K. and France for the Vietnam War?

2. France colonized Indochina and controlled it until they (Vietnam, Laos and Cambodia) became independent after the First Indochina War. France helped the U.S. in various wars including the American Revolutionary War, but didn't in the Vietnam War. Was there any particular reason?

Answer 1

The Korean War and the Vietnam War are fundamentally very different conflicts.

---

With the former, South Korea was unambiguously invaded by North Korea in an explicit war of conquest. Due to lucky political circumstances, the United Nations was able to sanction a military operation to defend South Korea.

The Security Council,

Having determined that the armed attack upon the Republic of Korea by forces from North Korea constitutes a breach of the peace,

(...)

Recommends that the Members of the United Nations furnish such assistance to the Republic of Korea as may be necessary to repel the armed attack and to restore international peace and security in the area.

_{- UN Security Council Resolution 83 (1950) of 27 June 1950}

Thus, the military forces of the free world rallied to South Korea under the banner of the United Nations. While many were also American allies, they joined the fighting not out of treaty obligations to the United States, but instead as member states of the United Nations.

---

In contrast, the Vietnam War was essentially a prolonged counter-insurgency operation within the territories of South Vietnam. There was no treaty to oblige American allies to participate in such a conflict. Nor was there any call to arms by the United Nations for defending South Vietnam.

The United States could ask and apply pressure on her allies for help. Nonetheless, with the lack of any real legal obligation, it fell to each government to decide whether participation was worthwhile. New Zealand, for example, was reluctantly pressured into sending a token force. Great Britain resolutely refused to officially participate, but sent assistance covertly - British soldiers ended up fighting in Australian and New Zealand units, for instance.

French arms had already fought in Vietnam before, and were bloodied by it. I don't think French public opinion could have supported a second trip to the quagmire especially now that they had no empire to fight for in Indochina.

Moreover, French-American relations were not the best during the 1960s. Around the same time the United States began committing significant combat troops, France was in fact withdrawing from NATO.

Answer 2

Let me add a few details to Semaphore's answer. One is that the Vietnam war actually ended with the North Vietnam invasion of South Vietnam. This happened soon after the US withdrawal, and in violation of the peace treaty. The UN Security Council could not react properly because the Soviets had the right of veto in it. (As it could not react in 2014 on the Russian invasion of Ukraine).

An interesting question is why the Soviets did not use their right of veto in the case of Korean war. It turns out that this was just a blunder of the Soviet UN delegation: when the discussion started, they just left the meeting as a protest.

It probably did not cross their minds that in this very meeting a decision on military action could be made. At the time of Vietnam, Soviet diplomats were already more experienced.

Answer 3

It's pertinent to note that several nations other than the U.S. participated in combat on the side of South Vietnam, with the largest contributions coming from South Korea, Thailand, and Australia. Many others gave civilian or military aid. South Korea deployed 50,000 soldiers over the course of the war, which is more than all the countries in the Korean War coalition (excepting the U.S. and Korea) combined. So while the particulars of international involvement were different, the reality on the ground was similar: majority of troops are from the country in question, most of the rest are from the U.S., and then a moderate percentage are from other nations.