HIMARS, a truck-mounted system which can fire missiles as far as 300 kilometers, represents a major boost to US combat power near At Tanf, a location that has come under the spotlight following a series of recent coalition strikes against pro-regime forces operating in the area.
While this is the first HIMARS deployment to southern Syria, a US official told CNN that this was not the first time HIMARS has been used in the country, with the system being deployed to the north to help the Syrian Democratic Forces in their offensives against ISIS. US Marines have also used M777 howitzers in support of SDF units fighting ISIS in northern Syria.
HIMARS had been previously used to strike ISIS targets from firing positions in Turkey and Jordan. The system has also been deployed in Iraq to hit ISIS positions there.
The area around At Tanf has become a flashpoint in recent weeks between pro-regime forces and coalition troops advising local forces battling ISIS.
Last week, a US F-15E jet shot down an Iranian-made pro-regime Shahed 129 drone that had dropped a "dud" munition near coalition forces patrolling near At Tanf, according to US officials. There were two additional US airstrikes against militia positions and vehicles inside the de-confliction zone around the base, plus another airstrike in May.
Following the first strike against pro-regime forces in May, a spokesman for the military coalition fighting ISIS said that the military was reinforcing its position there.
"We have increased our combat power in that area," US Col. Ryan Dillon told reporters at the Pentagon earlier this month.
"We have increased our presence and our footprint and prepared for any threat that is presented by the pro-regime forces," Dillon added.
One defense official said that the HIMARS move was a response to actions by pro-regime forces, who have been deploying their own artillery near the perimeter of the 55-kilometer "de-confliction zone" surrounding At Tanf. While the pro-regime artillery could not reach the base, it could reach a smaller combat outpost used by coalition advisers and their local allies.
A second official, however, said it was not clear whether the HIMARS deployment was in direct response to threatening action by the pro-regime forces.
CNN has previously reported that pro-regime forces had established a number of outposts and checkpoints in the area near the de-confliction zone. US officials have told CNN they believe pro-regime militias backed by Iran are setting up a series of checkpoints as part of an effort to establish a strategic overland route from Iran to the Mediterranean coast.
Privately, US military officials say they are hopeful matters will remain calm and no more US military action will be required. And publicly, senior officials have insisted that the clashes with pro-regime fighters are purely in self-defense and that the US and the coalition will remain focused on battling ISIS.
"Those are self-defense strikes and the commander on the ground has the authority to take whatever action necessary and I support that," Secretary of Defense Jim Mattis told the Senate Armed Services Committee Tuesday.