Hundreds of US troops are deploying to the Middle East on a mission to boost the odds of a moderate rebel victory in Syria’s civil war.

The news, reported on Thursday night, comes after Congress appropriated funds last December to help arm and train elements of the moderate Syrian opposition in their fight against both the Assad government and forces aligned with the Islamic state or al Qaeda.

According to Defense One, more than 400 troops will be dispatched to Turkey, Qatar, and Saudi Arabia to assist in training rebels, along with hundreds more to provide security at sites. The Pentagon is mum so far on the total troop deployment number, but they expect the mission will be a multi-year process.

Already the Syrian civil war has lasted nearly four years, and has claimed hundreds of thousands of lives. The Pentagon’s training and equipping regimen, is predicting that the war will last for at least another three years. In doing so, the Department of Defense is seeking to add 15,000 battle-ready soldiers to the conflict.

Navigating the labyrinth of opposition groups and their competing agendas in Syria will be difficult for US military officials. The Obama administration has long called for Syrian President Bashar al-Assad to step down first and foremost. But it is now giving moderate rebels the unenviable ask of defeating both the Assad regime and extremist rebel factions that have been allies in insurgents’ fight against Damascus.

For years, the US has been providing assistance to rebel groups, mostly in the form of communications systems, medicine, and food.

In late 2013, after allegations that the Assad government used chemical weapons, the US began working with Arab and European allies to funnel lethal aide to the Syrian opposition, sending guns, bullets, and money across the borders of Turkey and Jordan into Syria.

But the administration has been reluctant to provide heavy weaponry out of fear it could end up in the hands of fundamentalist rebel factions that have, so far, held the most significant swaths of territory in the uprising against the Assad government. Since the Islamic State seized control of large parts of Syria and Iraq, they consolidated their strength by taking possession of US military hardware originally provided to moderate Syrians and the Iraqi military.

The Sentinel reported Wednesday that former US Ambassador to Syria, Robert Ford, admitted that Washington knew much of its assistance in Syria was flowing into the hands of extremists because moderate rebels were willing to take them on as friends.

“The days of us looking the other way are finished,” he said, adding that “cooperation with a terrorist group as political impact.”

The Pentagon claims their process for vetting the opposition groups is improving, but remains hamstrung by lack of intelligence on the ground.

Also complicating oversight matters are US allies in the region who will be hosting the training sites. Last October, Vice President Joe Biden criticized Turkey and Saudi Arabia for not doing enough to stop the flow of manpower, money, and weaponry into Islamic State leaders’ hands.

Biden was later forced to apologize for his remarks.

The Pentagon’s train and equip program is ramping up after months of airstrikes against extremist targets in Iraq and Syria. Despite the heavy bombing campaign, the Islamic State in Syria has managed to expand its territory.