Arab country citizens do not need visas to enter Syria, but all others do. The Syrian Embassy grants single or multiple entry visas with a validity of 3 to 6 months, and travellers of most nationalities need a letter of recommendation with a no-objection certificate from their consulates to visit Syria. If you’re from a country that doesn’t have a Syrian embassy, you can obtain entry visas at land borders, but it’s better to do your homework beforehand.
US citizens who do not wish to go through the trouble of dealing with the red tape themselves can count on the professional services of Travel Visa Pro to speed things up: Apply for Syria Visa Online (service starts from $39). If you need to renew, add pages, change name, or just get a new US passport, you can Apply for a New US Passport here.