When the US Government determines that a visiting dignitary needs protection while visiting the United States, these services are most often provided by either the United States Secret Service

United States Secret Service

The United States Secret Service is a United States federal law enforcement agency that is part of the United States Department of Homeland Security. The sworn members are divided among the Special Agents and the Uniformed Division. Until March 1, 2003, the Service was part of the United States...

 or the US Department of State’s Diplomatic Security Service

Diplomatic Security Service

The U.S. Diplomatic Security Service is the federal law enforcement arm of the United States Department of State. The majority of its Special Agents are members of the Foreign Service and federal law enforcement agents at the same time, making them unique...

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Secret Service

In addition to US government protectees (POTUS, Cabinet members), the Secret Service

United States Secret Service

The United States Secret Service is a United States federal law enforcement agency that is part of the United States Department of Homeland Security. The sworn members are divided among the Special Agents and the Uniformed Division. Until March 1, 2003, the Service was part of the United States...

 protects visiting heads of state.

Diplomatic Security Service

In addition to US government protectees (SECSTATE, USUN Ambassador), the Diplomatic Security Service

Diplomatic Security Service

The U.S. Diplomatic Security Service is the federal law enforcement arm of the United States Department of State. The majority of its Special Agents are members of the Foreign Service and federal law enforcement agents at the same time, making them unique...

protects foreign dignitaries who are not heads of state, essentially everybody else.

In recent years DS agents have also protected foreign leaders while in their own country. This includes the president of Liberia, and the president of Afghanistan.