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.
Who violated the ceasefire under what circumstances is contested, but ultimately does not really matter - neither side wanted peace, and the hostilities would've resumed anyway one way or another.
The end result was that the US antagonized a people who were fighting for their own freedom, and found itself in an unwinnable war - creating an allied (or at least independent) country from a people you bombed to the stone age is an impossible task.
From this follows that no, the US military wasn't just another dozen bombed out villages away from final victory, when it was backstabbed by those peaceniks at home.
An ounce of diplomacy at the right place and time would've been the best for both the United States and Vietnam, and would've netted them everything they wanted on top of international goodwill.
Who resumed the fighting does matter, as even though Diem refused the 1956 reunification elections that didn't justify resorting to an armed insurgency by the North. It's also not ambiguous, Hanoi's 15th plenum officially authorised that insurgency in 1959 and formed groups 559 and 759, and later the NLF (Viet Cong).
They were already killing hundreds of innocent South Vietnamese village chiefs, teachers, and officials by the late 1950s, trying to create a local insurgency and insurrection. That's what the US got involved with to oppose, and I really can't say it was a morally wrong decision at the time. Hanoi had the option of sticking with diplomacy and chose to send guerrillas south instead. You're giving only the US agency here.