The State Department likes to call it the harbinger of both the CIA and the Department of Homeland Security.
One of the department's least-known operations emerged in the heated early days of World War I, when Secretary of State Robert Lansing was determined to hire someone to help him with security, espionage and intelligence threats, including the sensitive issue of tracking, expelling or interning diplomats from countries that were suddenly enemies. There was no CIA.
But wartime resources were tight in 1916, so the first "special agent" was paid out of Lansing's own pocket, according to State Department lore.
One agent soon grew to eight in the new Bureau of Secret Intelligence. But the quiet operation remained largely unknown even within government circles.
Nine decades later, with almost 1,400 special agents and a staff of 32,000, the renamed Bureau of Diplomatic Security has more agents deployed around the world than any other U.S. law enforcement agency. Its diverse and often dangerous missions are now on the frontline of the war on terrorism. Yet it remains little known -- deliberately.
At the recent Olympics, Diplomatic Security had about 100 agents embedded with the U.S. men's swimming team, women's gymnastics and several other teams, again more than any of the half-dozen or so U.S. agencies deployed in Athens.
During the 1995 raid in Islamabad, Pakistan, that captured World Trade Center bomber Ramzi Yousef, two agents were in the raid with Pakistani forces who seized him -- while the FBI waited outside. One agent had been the first to speak with the walk-in informant who identified Yousef's hideout. He then coordinated plans for the raid with Pakistani security forces.
Since the end of the U.S.-led occupation in Iraq on June 28, Diplomatic Security has taken over protection of the new U.S. Embassy in Baghdad, the largest diplomatic operation by any nation in the world.
It is also in charge of shielding about 7,000 American staff and many more relatives at the 265 U.S. embassies and consulates in 180 countries, an increasingly challenging mission since the 1998 bombings of the embassies in Kenya and Tanzania by al Qaeda. More than 200 were killed and more than 1,000 were injured in those al Qaeda attacks.
"The long tradition of diplomacy also has been marked by more sacrifice than most Americans will ever know. There are few professions more dangerous than the practice of foreign affairs, and there are few professionals who put more on the line for this nation than the agents of the Diplomatic Security service," Deputy Secretary of State Richard L. Armitage said last year.
In a role symbolic of its mission, one of the last people in the final helicopter to evacuate the U.S. Embassy during the fall of Saigon in 1975 was Leo Crampsey, Diplomatic Security's regional security officer, according to the State Department.
But its mission is far more than protection, U.S. officials say. Diplomatic Security is also in charge of investigations, from threats against the United States' diplomatic facilities overseas to visa and passport fraud, a central component in the war on terrorism.
Diplomatic Security's mission has expanded again recently to include the same service for foreign leaders. In Afghanistan, the bureau that normally protects Secretary of State Colin L. Powell and other high-profile U.S. diplomats is also now providing protection for President Hamid Karzai.
In Haiti, Diplomatic Security is protecting the interim president and prime minister while its agents train a new unit to replace them.