SOME jurisdictions require your passport to be valid for a period of six months past the date of entry, in order for you to gain entry into their country. The issue is NOT coming back into the US, it is entering the foreign country. What that means is that the airline could well deny you boarding on the airplane when you leave home. Apparently that is not the case with Sint Maarten. However, once there, you may not be able to travel to other countries, such as St. Barts, which I think perhaps has the six month rule.