Question

why did american presidents feel it was important to help south vietnam?

if they allowed vietnam to become communist it may cause a domino effect in southeast asia.
north Vietnam has threatened to use uclear weapons on the usa.
the soviet union was supportive of south vietnam too.
north vietnam threatened to take over cambodia and laos also.

Answer 1

Short answer: Because U.S. leaders saw Vietnam as a Cold War domino — allowing a communist takeover in Vietnam seemed likely to encourage communist gains elsewhere, weaken U.S. credibility with allies, and strengthen the Soviet/Chinese communist bloc. That combination of strategic, political and ideological concerns made successive presidents decide to support South Vietnam.

More detail, and corrections to the points you listed:

Main reasons U.S. presidents acted to help South Vietnam
- Containment and the “domino theory”: U.S. policy after World War II aimed to stop the spread of communism. Many American leaders believed that if Vietnam fell to communism, other countries in Southeast Asia might follow, hurting U.S. interests and regional allies.
- Credibility and alliances: Presidents worried that abandoning an ally would damage U.S. credibility with partners (and encourage insurgencies elsewhere). Treaty obligations and agreements such as SEATO shaped thinking.
- Cold War competition: The U.S. saw Vietnam as part of the global contest with the Soviet Union and China. Helping South Vietnam was a way to limit Soviet and Chinese influence in the region.
- Regional stability and anti-insurgency policy: U.S. leaders feared that communist victory in Vietnam would support insurgent movements in neighboring Laos and Cambodia and destabilize the region.
- Domestic politics and leadership concerns: Presidents also faced domestic political pressure not to appear “soft” on communism; fear of political consequences influenced decisions to escalate or maintain commitments.

Corrections to the statements you listed
- “If they allowed Vietnam to become communist it may cause a domino effect in southeast Asia.” — This was a central rationale and widely held by U.S. policymakers (the domino theory).
- “North Vietnam has threatened to use nuclear weapons on the USA.” — Not accurate. North Vietnam did not possess nuclear weapons and did not credibly threaten nuclear attack on the United States. The nuclear threat in the crisis was tied to the major powers (U.S., USSR, China), not Hanoi.
- “The Soviet Union was supportive of South Vietnam too.” — Wrong. The Soviet Union (and communist China) supported North Vietnam politically, economically and militarily. The United States and its allies supported South Vietnam.
- “North Vietnam threatened to take over Cambodia and Laos also.” — Partly correct in the sense that North Vietnam supported communist insurgents in Laos (the Pathet Lao) and had forces and influence in Cambodia; U.S. leaders feared communist expansion and insurgency in those neighboring states.

Brief timeline context
- Truman/Eisenhower: support for French and then the anti-communist South as part of containment.
- Kennedy: increased advisers and special forces to bolster South Vietnam.
- Johnson: major escalation after the Gulf of Tonkin incidents (which later came under question).
- Nixon: pursued “Vietnamization” (shift combat to South Vietnamese forces) while negotiating and expanding bombing into Laos/Cambodia to block North Vietnamese supply lines.

If you want, I can give a short timeline of each president’s decisions, or sources to read for more detail.