Owing to complex bureaucratic rules, the acting defense secretary of the past four weeks, Mark Esper, was forced to step down as Trump submitted his formal nomination for the permanent job to Congress.

While his nomination is under consideration in the Senate, Esper will fall back to his former job as secretary of the army, and current Navy Secretary Richard Spencer will assume the role of acting secretary of defense.

"While my time in this role is anticipated to be brief, I am fully prepared and committed to serve as acting secretary of defense, and will provide continuity in the leadership of the department," Spencer said in a letter addressed to military personnel.

Esper, 55, is expected to be easily passed by the Senate Armed Services Committee, which will hold a hearing on his nomination on Tuesday.

Then the full Senate must confirm the nomination in a vote that could take place before the end of the month.

The deeply respected Mattis resigned in December in a break with Trump over policy toward the Middle East and Afghanistan.

Trump then nominated Mattis' deputy secretary of defense, Patrick Shanahan, as a replacement.

But Shanahan, a former Boeing executive who had never served in the military, ran into resistance in the Senate and finally withdrew after an FBI background check revealed a violent altercation with his then-wife in 2010.

The Pentagon has never gone without a confirmed secretary of defense for so long.

The gap in permanent leadership was more acute as tensions rose with Iran and Trump flirted with a military strike against the country last month.

Unlike Shanahan, Esper served in the 1991 Gulf War and was part of the Army's famous 101st Airborne Division, recently honored at the commemoration of the 75th anniversary of the D-Day landings at Normandy.

He is also close to top US diplomat Mike Pompeo, with whom he studied at the prestigious West Point military academy. Both men graduated in the same year, 1986.