Below are some suggestions for organizations to check out for internship opportunities. 

Gilder Lehrman Institute (NYC)
A summer internship as well as one week seminar for undergraduates

Cold War International History Project (DC)

Navy History Center Internships (DC)

State Department (DC)
Very early application, usually in fall

Center for Strategic and International Studies (DC)

World Teach (Cambridge, MA)
International education; jobs teaching English and the like in foreign countries

Jane Goodall Institute (Boston, MA)
Seeking candidates for a part-time volunteer internship. Get details