Canada2017:
"Vietnam began as 'weapons support '….then 'advisors on the ground '….then finally 500,000 troops deployed ."That's both an important lesson, and yet also a trap leading to false assumptions.
Vietnam started back during the Eisenhower Administration, and carried through the Kennedy and Johnson Administrations into Nixon's time. The final collapse of South Vietnam, of course, was the doing of the Democrats while Ford was President. There was a viable plan in place which the Congress abandoned, knowing Ford lacked the power to protect the South Vietnamese from invasion.
One of the underconsidered lessons from the Vietnam conflict, is that to a degree the Domino Theory was right. The loss of South Vietnam led to trouble for the entire region. In the long term, it would have been better for the world if the US had planned for more than just a war which goals and commitment changed according to the public mood.
Russia, historically, has always seen its neighbors as targets for acquisition. Various excuses have been offered to justify the wars, but the only times Russia has held back from wars of aggression, is when they have a means of prosperity tied to peace, or when strong nations have made clear that war would not succeed.
That means that with regard to Ukraine, the US will have a lot to do with the stability of Europe in its decisions. Germany and France, in particular, have shown they will cow to Russia when no one appears ready to stop the Russians.
But no one is saying we should commit US troops. Biden, foolish as he is, understands not to cross that line.
And I do agree that there is a limit to how much in weapons we should donate to Ukraine. At some point there needs to be a discussion about where we call an end to being Ukraine's arsenal.
That which does not kill me, will try again and get nastier