How HR McMaster's military rank could affect his role as national security adviser

— -- The naming of Army Lt. Gen. H.R. McMaster as President Trump's new national security adviser has raised some questions about how an active duty officer can serve in the job.

It has been done before. McMaster will be the fourth active duty military officer to serve as national security adviser.

There are some challenges that an Army three star general could face in the role while remaining on active duty.

ABC News looks at some of the potential issues facing McMaster.

The National Security Council

The job of the national security adviser evolved in the 1950s from the 1947 National Security Act, which created the National Security Council. The adviser oversees the council, which coordinates policy among the Pentagon, the State Department, the intelligence services and other government agencies involved in national security. The adviser is typically one of the president's closest aides, and the position does not require Senate confirmation.

Originally a small staff, over the last few decades the National Security Council has ballooned in size to several hundred employees.

Its staffers are typically detailed from the military or relevant agencies to work in their field of expertise.

National security advisers have typically been civilians, though three military officers previously served in the role.

In late 1975, when Brent Scrowcroft was named as Gerald Ford's national security adviser, he was an active duty Air Force lieutenant general who was serving as the deputy national security adviser. But a month after being named to the top job, Scrowcroft retired from the Air Force because he felt the job should be held by a civilian. He continued in the post through the end of the Ford administration as a civilian.

Navy Vice Adm. John Poindexter and Army Lt. Gen. Colin Powell remained on active duty when they served as national security adviser for Ronald Reagan.

Though Senate confirmation is not required for the post, Powell had a confirmation hearing so he could retain his three star rank.

The White House said McMaster will remain on active duty during his tenure. But if he wants to serve in the role as a three star general, a Senate Armed Services Committee aide told ABC News Tuesday, "the law requires that Gen. McMaster would have to be reappointed by the president and [confirmed] by the Senate in that grade for his new position."

Otherwise, to avoid Senate confirmation, the aide said, McMaster "could serve as national security adviser in his permanent rank of major general [two stars] or retire. Neither of those require any Senate action."

McMaster's role

Judging by the service of his active duty predecessors, McMaster's rank will not be an impediment in dealing with officers of superior rank.

McMaster's main task will be to coordinate the administration's foreign policy and national security decisions among the relevant agencies of the federal government.

His rank will not affect their relationship to the national security adviser, who reports directly to the president. That level of access will allow McMaster to voice his opinions freely and directly to the commander in chief, which should not be a stretch for McMaster, who is known as an independent thinker willing to speak his mind.

He is also known as a noted military strategist and pioneer in the counterinsurgency doctrine that helped turn the tide in Iraq.

McMaster's military advancement once seemed to have stalled — namely after the publication of his 1997 book, "Dereliction of Duty," which criticized military officers for not challenging political decisions during the early years of the Vietnam War.

His career path regained an upward trajectory after success in stabilizing the northern Iraqi city of Tal Afar and developing counterinsurgency strategy.

In his new role, McMaster will probably demonstrate some of his candor with respect to Russia, whose military moves in recent years he has viewed warily.

ABC News' Ali Rogin contributed to this article.