President-elect Trump's nominee to lead the Department of Homeland Security, retired Marine Corps Gen. John Kelly, sits for a confirmation hearing Tuesday afternoon, facing questions from members of the Senate Committee on Homeland Security and Governmental Affairs.

The 66-year-old Kelly has appeared before lawmakers before, in his role as head of the U.S. Southern Command, which is responsible for military matters in the Caribbean, Central and South America. Kelly has also served as an adviser to former Defense Secretaries Leon Panetta and Robert Gates. CNN reports Gates will introduce Kelly at the start of the hearing along with Republican Sen. John McCain.

The Department of Homeland Security is a sprawling enterprise, with a budget of some $40 billion and a quarter-million employees. Its responsibilities encompass everything from natural disasters (FEMA), to airport security (TSA), presidential security (U.S. Secret Service), the Coast Guard and cybersecurity.

DHS agencies are also responsible for defending the nation's borders and overseeing the immigration system. It's those areas that are most in the sights of the incoming president, who has called for measures including a wall along the U.S. border with Mexico, the deportation of millions of undocumented immigrants currently in the U.S. and "extreme vetting" of immigrants wishing to enter the country.

Trump said on the campaign trail that should be focused on those coming from "some of the most dangerous and volatile regions of the world that have a history of exporting terrorism," as an adaptation of the ban on Muslims entering the U.S. Trump proposed at the end of 2015.

Senators are likely to question Kelly closely on those issues. Kelly has stated that he believes economic aid and education are as necessary to stop the flow of migrants from Central America to the U.S. as a strong barrier. He has also stated that he believes half a million Americans have died from narcoterrorism.

Kelly's nomination is unlikely to be met with much opposition. He has built up relationships with lawmakers who respect his service and his sacrifice. (Kelly lost a son to combat in Afghanistan.)

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