The US only became actively involved in supporting South Vietnam a decade after it had achieved independence from France in 1955, after the negotiated division into North and South Vietnam in Geneva, and after the last French forces left in 1956.

The cause of that intervention was North Vietnam sending Viet Cong insurgents into South Vietnam, attempting to overthrow the government, and ignoring the South's sovereignty.

That doesn't justify the US's later transgressions while engaged in the war, but their initial support to South Vietnam was neither unjustified nor illiberal.