WASHINGTON — Virginia Sen. Tim Kaine is demanding the Pentagon provide more details on the US military’s role in Niger and explain how differentiates between “advise and assist” and combat missions.

The 2016 Democratic vice-presidential nominee sent a letter to Defense Secretary James Mattis on Monday asking for more information about US troops deployed to Niger and other countries where they are training foreign forces, following the October 4 incident in Niger in which four US service members were killed.

The questions Kaine raised are already on the mind of military leaders as the Army is considering revamping how it trains foreign militaries.

“While I fully appreciate both the necessity and importance for our armed forces to assist in the professionalization and capacity building of local security forces around the globe, to include those in Niger, I am concerned that our complex operating environment has made it nearly impossible to differentiate between “advise and assist” and combat operations,” Kaine wrote.

Kaine’s letter, obtained by CNN, is the latest instance where lawmakers are pressing the Pentagon to provide more details on the circumstances surrounding the October 4 Niger incident, in which four US service members were killed.

Senate armed services chairman John McCain has accused the Pentagon of not being forthcoming with details surrounding the incident, and both his committee and the House armed services panel are expected to receive briefings on Niger on Thursday.

Chairman of the Joint Chiefs Gen. Joe Dunford spoke to reporters at the Pentagon Monday about the Pentagon’s investigation into the ambush.

The Niger incident is sparking new questions on Capitol Hill about the US military’s role in Africa, and proponents of a new war authorization for ISIS are pointing to it as another sign that the current 2001 Authorization for Use of Military Force is outdated.

Kaine has been one of the most vocal advocates in Congress calling for a new war authorization. He argued in his letter that the blending of advise and assist missions with combat operations “makes the line triggering the requirement for congressional authorization and approval blurry.”

“Specifically, since 2013, the US military personnel presence in Niger has grown from approximately 100 personnel to over 800 today, making Niger host to one of the largest US troop presences in Africa and raising the question of ‘mission creep,'” Kaine wrote.

The Army is already in the process of revamping how it trains foreign militaries, a skill seen as critical to achieving lasting success on the counterterrorism battlefields of Syria, Iraq, Afghanistan and elsewhere.

While it’s a critical component of US strategic objectives in the fight against terrorism, critics say the Army has historically not prioritized training and advisory missions.

It’s a failure and missing skill set that the Army’s leadership acknowledged on the sidelines of the Association of the United States Army’s annual meeting in Washington earlier this month.

“What we’ve done for 16 consecutive years is done that function on essentially an ad hoc basis,” Gen. Mark Milley, the Army Chief of Staff, told reporters in a press conference at the meeting.

To address the issue, the Army in May moved to establish the first of what it is calling “Security Force Assistance Brigades” specifically designed to more effectively train foreign militaries battling terrorist groups.

“One of the initiatives that we the Army are doing to increase our capability to conduct train, advise and assist, enable and accompany missions around the world is the establishment of Security Force Assistance Brigades,” Milley said.

“One of the reason past efforts have failed they were temporary in nature,” Milley explained, noting that training teams were created ad hoc by pulling officers from regular brigade combat teams which he said undermined the latter’s readiness.

The Army plans to create six new Security Force Assistance Brigades, five for the regular army and one in the National Guard, with each brigade consisting of about 500 soldiers.

“We are likely to be involved in train, advise, assist operations around the world for many years to come,” Milley said, adding “so we think it’s about time we recognize that fact with force structure.”

The US Army Special Forces, commonly known as the “Green Berets,” were created during the early days of the Cold War, with one of their key missions being the training the military and police forces of America’s Cold War allies to battle enemy insurgencies.

And while US Special Operations Forces from across the military services in recent years have worked closely with local partners, helping the Afghan Commandos and the Iraqi Counter-Terrorism Service become two of the most effective battlefield American allies, training efforts for less elite partner troops were considered less effective and local conventional forces have been seen as less capable.

Effective training could also relieve some of the strain being placed on US special operations forces who have become increasingly central to a variety of US military missions across the globe.

The “mission frankly is bigger than Special Forces has ever been designed or capable to handle,” Milley said of the train and advisory mission.

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