This isn’t the same country it was before 1865. You couldn’t even imagine a Vietnam-like scenario in, say, 1840 … where the President unilaterally pissed away 50,000 American lives in a military campaign halfway around the world with no Congressional declaration of war.
My primary objection is to your comment that “nothing really stops an imperial/totalitarian president.” More to the point, the Supreme Court rejected challenges to the Vietnam War based on the war powers clause because Congress funded the war. As it was, when Congress cut off funding, the US role in the Vietnam War ended.