The US should have a permanent human presence on the Moon, and do it first.
The US should only send a human mission to Mars if it plans to plant the US flag.
The US should not engage in any more “international” efforts in space.
That said, the only two reasons to go back to the Moon are to A) establish and maintain a permanent base (or more than one) on the Moon, and B) to develop and practice the skills and technology to make very long duration spaceflights (to Mars, obviously) achieveable.
The reasons for a lunar base are probably numerous, but in particular should be done to A) enable human missions to Mars, B) “far side” radiotelescopy, mostly for the identification of solar system debris potentially threatening to Earth, C) assay the lunar surface for useful materials which are rare on Earth, and D) build a giant [quote fingers] laser [unquote fingers] to blackmail the OPEC nations.
I would like to see the US go to Alpha Centauri first and plant a flag there..