Under President Donald Trump, the United States' role is likely to expand further.
While Trump has announced no changes to the U.S. approach, the Pentagon in recent months has incrementally increased its footprint in the northern reaches of the war-ravaged, Middle East country, where it is backing a coalition of Syrian Arab and Kurdish fighters closing in on the Islamic States' self-declared capital at Raqqa.
Trump is reviewing options for accelerating the recapture of Raqqa. These include proposals for more U.S. troops, greater firepower and tweaks in the existing strategy.
Here is a look at how the U.S. mission has evolved, how it stands today and challenges facing the Trump administration it contemplates speeding up the fight.
HOW IT BEGAN
Former President Barack Obama ordered the start of a U.S.-led air campaign against ISIS in Syria in September 2014, weeks after a parallel bombing effort began in neighboring Iraq.
IS militants that year had swept across Syria's border into northern and western Iraq, capturing the city of Mosul and declaring an Islamic caliphate. Its rapid progress created alarm in Washington and around the world about the prospect of Baghdad potentially falling.
Almost 1,000 days later, the Pentagon says it has spent $11.5 billion. That includes money for training and advising local forces.
Obama initially ruled out putting U.S. ground troops in Syria, but sent small numbers of military advisers to Iraq to develop a plan for retraining an Iraqi army that had all but collapsed. Gradually the U.S. role in Iraq widened and deepened. There are now well more than 5,000 U.S. troops in Iraq, largely focused on helping the government recapture Mosul.
Later, Obama authorized an initial contingent of about 200 U.S. special operations troops into Syria. Their task was to recruit and organize local fighters who were opposed to the Syrian government of President Bashar Assad, but willing to prioritize the battle against ISIS in its strongholds in the country's north. Last December, Obama boosted the U.S. troop total in Syria to as many as 503.
In recent weeks, under Trump, U.S. forces in the country climbed to roughly 1,000.
THE CURRENT MISSION
The mission in Syria is being carried out by U.S. special operations troops as well as several hundred conventional forces such as a Marine artillery unit.
The Americans aren't leading the fight against ISIS but are involved in an increasing number of ways. The Marine artillery is a recent addition, for example, and about two weeks ago a few dozen Army Rangers began acting as a "deterrence and reassurance" force on the outskirts of the city of Manbij. The Rangers are showing the U.S. flag in hopes of dissuading Turkish, Russian, Syrian and U.S.-based opposition forces from fighting each other, deliberately or accidentally.
The air war continues. The U.S. is conducting strikes on ISIS daily from bases in Jordan, Turkey and elsewhere in the region.
The Marines in Syria arrived from the USS Makin Island and two other ships that moved into the region last November. The ships carry the 11th Marine Expeditionary Unit of about 2,400 Marines. They're expected to leave and eventually be replaced by the 24th Marine Expeditionary Unit that left Norfolk, Virginia, in early March with the USS Bataan, USS Mesa Verde and USS Carter Hall.
Commanders have raised the prospect of sending additional forces into the region to be ready to assist in accelerating the fight. They could head to either Syria or Iraq.
But decisions have largely stalled. Discussions continue within the Trump administration on possibly ending strict limits on troop numbers set by Obama. The new approach would give commanders more flexibility in determining how many forces they need.
THE WAY AHEAD
Military commanders, frustrated by what they considered micromanagement under the previous administration, have argued for greater freedom to make daily decisions on how to fight the enemy. And Trump says the threat must be extinguished quickly.
Still, America is having some success at the moment. U.S.-trained Iraqi forces have pushed ISIS to the brink of a major loss in Mosul, following the group's defeats in the western Iraqi cities of Ramadi and Fallujah.
But the elimination of ISIS in Iraq appears far from over, and U.S. commanders may determine they need more troops there.
It's an altogether tougher situation in Syria, where the fight against ISIS is happening simultaneously with Syria's six-year-old civil war that has killed hundreds of thousands of people.
Turkey, a U.S. NATO ally, is working at cross-purposes with the U.S. in some respects. Turkey's military incursion into northern Syria aims to push ISIS militants back from the border and prevent Kurdish forces from holding contiguous territory on Turkey's frontier.
Turkey considers the main Syrian Kurdish force, the YPG, as terrorists because of their links to Kurdish insurgents in Turkey. Washington, on the other hand, considers the YPG its most effective partner on the Syrian battlefield.
The Pentagon would like to arm the Syrian Kurds as part of a planned push to recapture Raqqa, but the Turks are adamantly opposed. Efforts to reach some type of compromise have dragged on for months. So far, they've been unsuccessful.
Any significant increase or change in military action in Syria will be tough to advance while questions related to Turkey and the Kurds remain unsettled.