Mark Esper is the third man to lead the Pentagon in six months.

Trump's new pick for Pentagon chief Mark Esper takes over as acting defense secretary on Monday -- and will quickly find himself in the hot seat on everything from tensions with Iran to Washington's position within NATO.

Esper, the secretary of the US Army who served in its much-heralded 101st Airborne Division, will bring the soldiering experience to the Department of Defense that former acting chief Patrick Shanahan lacked.

But his years of work as a lobbyist for defense firm Raytheon may spark concerns on Capitol Hill about the awarding of weapons contracts -- an issue that also plagued Shanahan because of his ties to Boeing.

President Donald Trump on Friday formally nominated the 55-year-old Esper, three days after Shanahan withdrew his name from consideration after past episodes of family domestic violence resurfaced during his background check.

"I know Mark, and have no doubt he will do a fantastic job!" Trump tweeted.

Esper, who must be confirmed by the Senate, is the third man to lead the Pentagon in six months, after Shanahan and Jim Mattis, Trump's first defense secretary who resigned in December last year.

For some Democrats, the multiple changes at the Defense Department have left a power vacuum, at a moment when Trump nearly authorized strikes on Iran over the downing of a US surveillance drone.

"This is a particularly fraught time for there to be not only an acting but an acting-acting," Democratic Senator Chris Coons told political news outlet Roll Call.

Military and political experience

Esper comes from a top military pedigree -- he graduated from the US Military Academy at West Point in 1986, and served as an infantryman, including during the Gulf War in 1990-91.

After his time on active duty, he served in the National Guard and Army Reserve before retiring from the military in 2007.

The married father of three adult children also has Master's degrees in government and public policy -- and years of experience on Capitol Hill, both as an congressional aide and as a lobbyist.

He is close to Secretary of State Mike Pompeo, a West Point classmate, and National Security Advisor John Bolton -- both outspoken hawks on Team Trump.

He met Bolton while serving as the deputy assistant secretary of defense for negotiations policy, focusing on arms control.

"Mr Esper's distinguished military service and record of leadership at the Pentagon make him an outstanding choice," said Republican Senate Majority Leader Mitch McConnell.

Multiple issues to tackle

But it will be a significant shift to move from being the Army's civilian chief tasked with manpower and procurement issues to devising overall defense policy for the world's most powerful military.

Esper is due to make his international debut on June 26-27 at a meeting of NATO defense ministers in Brussels.

One issue to confront there is a dispute with alliance ally Turkey over Ankara's purchase of Russian S-400 missiles. The US has threatened sanctions if Turkey goes ahead with the deal.

Trump has also said he is considering sending 2,000 troops to NATO ally Poland, from fellow alliance member Germany.

But he's already front and center, as Trump considers his options with Iran.

"All I want is no nuclear weapons," the Republican leader said Saturday.

Esper is expected to be easily confirmed in the Senate, after his 89-6 approval by the chamber for the Army secretary job in November 2017.

But Democrats and government watchdog groups say they will be listening carefully to his testimony for any red flags.

"Esper's vetting should be airtight. I look forward to asking him questions about his national security strategy & industry ties," tweeted Democratic Senator Richard Blumenthal, who serves on the Armed Services Committee.

"After months of an acting SecDef, sound & strong leadership at the Pentagon is critical."

Citizens for Responsibility and Ethics in Washington voiced concerns about his lobbying work, saying: "We'll be closely watching his ethics agreement -- and his ability to follow it."