The bureau of diplomatic security (DS) is the security and law enforcement arm of the U.S. Department of State and is responsible for providing a safe and secure environment for the conduct of U.S. foreign policy.
Every U.S. diplomatic mission operates under a security program designed and maintained by Diplomatic Security. In the United States, Diplomatic Security personnel protect the Secretary of State and high-ranking foreign dignitaries and officials visiting the United States, investigate passport and visa fraud, and conduct personnel security investigations.
Outside of the United States, Diplomatic Security ensures that America can conduct diplomacy safely and securely. DS plays a vital role in protecting U.S. embassies and personnel overseas, securing critical information systems, investigating passport and visa fraud, and fighting terrorism. Usually, local security guards are integrated into DS protection plans.
Marine Security Guards, working in coordination with DS personnel, protect many U.S. diplomatic and consular faculties abroad. They provide internal security to prevent the compromise of classified material vital to U.S. national security. They also provide protection for U.S government property and U.S. government employees located within U.S. diplomatic and consular premises, especially when under attack or during other urgent circumstances. (Taken from Marine Corps Embassy Security Group website).