Although each country is different, many international destinations do not allow non-citizens to cross their borders without a passport that is valid for at least six months after entry. Some only enforce a 3-month expiration rule. Still others require merely that the passport is valid for the duration of the journey.
Laws are subject to change at any time, so always check for updated rules. The information for each country is located on the U.S. Department of State's travel website. As of 2014, countries requiring a passport to be valid for six months include, among others, Brazil, China, Singapore, Thailand, the Bahamas, Jamaica, St. Lucia, Malaysia and the Philippines.
Do not attempt to travel with a passport that expires within six months because you may be turned down at the airport upon check-in or denied entrance when reaching your destination. If you discover at the last minute that your passport is not valid for the required amount of time, request an expedited passport renewal. This still takes at least a few days, so go directly in person to an official passport agency location, or hire a private expediting company to streamline your application. Fees are typically high for this service.