Usually, an ordinary passport is valid for 5 to 10 years, and because of this, most people neglect to check if their passport is about to expire. However, if your passport has less than 3 or 6 months before expiration, most countries do not allow you to travel.
Spain is a party to the Schengen Agreement. This means that U.S. citizens may enter Spain for up to 90 days for tourism or business without a visa. Your passport should be valid for at least three months beyond the period of stay. You must have sufficient funds and a return airline ticket.
Your passport should be valid for at least three months beyond the period of stay if you plan on transiting a Schengen country. Please review our U.S. Travelers in Europe page. We recommend that your passport have at least six months' validity remaining.
While Greek law requires three months of passport validity beyond the intended date of your departure, if you are transiting a country that requires six months of validity, you may not be permitted to continue your trip. We recommend that you have at least six months of validity to avoid any travel disruptions.
If your passport is about to expire, please take note of the following travel requirements: Entry into any of the European countries in the Schengen Area for short-term tourism, business trips or in transit to a non-Schengen destination requires your passport be valid for at least three months beyond your intended date ...
U.S. nationals, including U.S. dual nationals, must use a U.S. passport to enter and leave the United States. U.S. dual nationals may also be required by the country of their foreign nationality to use that country's passport to enter and leave that country.