The US didn't directly overthrow the government of South Vietnam, but the US did tell the South Vietnamese army that they would lose funding and support if they

The US didn't directly overthrow the government of South Vietnam, but the US did tell the South Vietnamese army that they would lose funding and support if they didn't overthrow Diem in 1963. Given that the army of South Vietnam was basically an appendage of the American military at that point it was a threat the South Vietnamese generals couldn't afford to ignore.