I think in both Vietnam and Korea we committed military forces without an intention to win, and indeed with a feeling that we could achieve some of our objectives simply by having some men there who would participate in limited kinds of activity, but in the case of Korea told not to win, and in the case of Vietnam clearly without the intention of applying the degree of force necessary to win. That's a very terrible thing to do to military people – to ask them to go into extreme danger, but in effect to have a decision made that it's not important enough for us to win. —Caspar Weinberger,
Secretary of defense in Reagan administration,
1987

What reason does Weinberger give for the United States losing the war?

The United States had never fought in a jungle setting before.
The United States’ troops were not strong enough to fight.
The United States entered the war without a real commitment to the conflict.
The United States should have followed the same approach as in the Korean War.