Originally Posted by sbm12

Continental.com has a great resource from Timatic regarding travel documentation; it is very accurate and is maintaind by people where that is their job, rather than Wikipedia where it is a bit of a crapshoot.

For a US citizen embarking in the FSM and arriving in the USA it has the following to say:



So while you may be able to prove your legal right to enter the USA in other ways, a passport is the easiest way to do so and will have the most consistent (and likely positive) results.