John Kelly, the retired Marine Corps general picked Monday by Donald Trump to be Homeland Security secretary, will take the lead in the new administration's promised fight against Islamic extremists and illegal immigration.

The third former general chosen for President-elect Trump's cabinet, Kelly capped a 45-year military career as head of the US Southern Command, an assignment that immersed him in border security issues, migrant flows and counter-drug operations in the Caribbean and Central and South America.

That experience -- and his record running large organizations -- will be useful if he is confirmed by Congress to take charge of the Department of Homeland Security, a sprawling bureaucracy responsible for border protection and internal security.

With 240,000 civil servants, Homeland Security includes the Secret Service, the Coast Guard, immigration and customs enforcement, and the agency that protects airports.

It also has an intelligence arm and an office dedicated to preventing nuclear terrorism.

Those functions would make Kelly a pivotal figure in carrying out Trump's election promises to build a wall on the Mexican border, deport undocumented immigrants and tighten legal immigration processes to screen out potential extremists.

The blunt-talking Kelly is close to retired general James Mattis, Trump's nominee for Secretary of Defense. Kelly served as Mattis's top aide in the 2003 assault on Baghdad that crushed Saddam Hussein's army.

He is also shaped by the experience of having his own son, also a Marine, die in battle. First Lieutenant Robert Michael Kelly was killed in action in Afghanistan in 2010.

A native of Boston, Kelly, 66, enlisted in the Marines when he was 20, spent two years in an infantry company, and then left to go to university.

After graduating he rejoined the Corps as a second lieutenant and rose steadily through a number of positions, including company and battalion commands.

In the late 1990s, he was the Marine Corps commandant's liaison to Congress, the first of two such stints that exposed him to the political give-and-take between the military and legislators.

In 1999, he began a two-year stint as special assistant to the Supreme Allied Commander, Europe, in Mons, Belgium.

In 2002-3, as a colonel and then brigadier general, Kelly returned to the 1st Marine Division, serving under then major general Mattis in the successful assault on Baghdad.

Five years later he was back in Iraq for the US occupation, holding two posts over the 2008-2011 period. He earned his fourth star when he was named to head the US Southern Command in 2012, a position he held until retiring in January 2016.

Besides contending with violent drug gangs operating across the region and deep inside the United States, Kelly also was responsible for the prison for war on terror detainees at Guantanamo Bay, Cuba.

The facility still houses 59 men, including the alleged masterminds of the September 11, 2001 attacks, despite President Barack Obama unfulfilled pledge to shut it down.

- Massive security bureaucracy -

Managing the Department of Homeland Security, which was established after the 9/11 attacks, will be a massive task in itself.

Representative Michael McCaul, chairman of the House Homeland Security Committee, recently said it needs a sweeping overhaul.

"It's time to undertake wholesale reform at DHS, including eliminating bureaucratic bloat, scrapping failing offices, and cutting through the red tape," he said in a speech at the Heritage Foundation conservative think tank.

"We must make sure DHS stays ahead of our enemies and that it leverages private sector innovation to deal with emerging threats from drones, IEDs, and beyond."