WASHINGTON — Massive U.S. airstrikes on Islamic State militants in Syria were meant in part as a message to the group and a move to ensure that it doesn’t try to take advantage of the chaos following the overthrow of President Bashar Assad’s government.

The U.S. and its partners want to make sure the Islamic State group, which still has a presence in Syria, can’t step into the leadership void and once again exert control over wide swaths of the country, Pentagon spokeswoman Sabrina Singh said Monday. The U.S. on Sunday struck about 75 IS targets in the Syrian desert.

The U.S. has had troops in Syria for the last decade to battle IS. The tumult following a rebel offensive that toppled Assad has raised fears of an Islamic State resurgence.

“ISIS will try to use this period to reestablish its capabilities, to create safe havens,” Secretary of State Antony Blinken said Monday, using another acronym for the group. “As our precision strikes over the weekend demonstrate, we are determined not to let that happen.”

So far, U.S. officials are saying that they do not plan an increase in American forces in Syria but are focused on making sure those already there are safe.

Here’s a look at the U.S. fight against the Islamic State group:

What’s the U.S. military presence in Syria?

The U.S. has about 900 troops and an undisclosed number of contractors in Syria, largely at small bases in the north and east, with a small number farther south at the al-Tanf garrison closer to the Iraq and Jordan borders.

U.S. special operations forces also routinely move in and out of the country but are usually in small teams and are not included in the official count.

Islamic State militants seized large parts of Iraq and Syria in 2014, declaring a caliphate. The U.S. gathered a coalition of allies and was able to defeat IS in Iraq in 2017. The U.S. partnered with the Kurdish-led Syrian Democratic Forces, or SDF, and after fierce fighting, ultimately declared an end to the caliphate in Syria in 2019.

Remnants of the militant group remain, including as many as 10,000 fighters held in SDF-run detention facilities in Syria and tens of thousands of their family members living in refugee camps.

And IS fighters have been more active over the past year or so, including in attacks against U.S. and Kurdish forces in Syria.

The country has been wracked by violence and competing interests. Russia has a naval port in the north, and while there have been fewer Russian forces in the area since the onset of the war in Ukraine, the U.S. maintains a deconfliction phoneline with Moscow to avoid any troop miscalculations on the ground or in the air.

Iran also has had a significant presence, often using Syria as a transit route to move weapons into Lebanon for use by Hezbollah militants against Israel.

The al-Tanf garrison in southeastern Syria is located on a vital road that can link Iranian-backed forces from Tehran all the way to southern Lebanon and Israel’s doorstep. So troops at the U.S. garrison can try and disrupt those shipments.

Why is the U.S. striking Islamic State targets?

The U.S. has, over the past decade, routinely targeted IS leaders, camps and weapons in Syria to keep the group at bay and prevent it from coalescing.

In the past year, as Israel’s war with Hamas widened into a broader conflict with Hezbollah in Lebanon, attacks by Iran-backed militias in Iraq and Syria as well as by the Islamic State group have escalated.

As a result, the U.S. has kept up a steady drumbeat of counterattacks against all the groups, including against IS camps in the desert, where fighters found safe haven.

Officials say that while the group is vastly weaker than in 2014, it still maintains thousands of militants in Syria.

On Sunday, the U.S. launched one of its larger, more expansive assaults against IS camps and operatives in the desert, taking advantage of the Assad government’s downfall. The U.S. bombed at least 75 targets in about five locations using B-52 bombers, A-10 attack aircraft and F-15 fighter jets.

“Does it send a message? I mean, I think it absolutely sends a message that we use B-52s, A-10s and F-15s,” Singh told reporters. She had no other details on the result of the strikes.

What’s next for the U.S. in Syria?

The Biden administration insists the U.S. will not get involved in Syria’s war or the overthrow of the Assad government. But the U.S. and its allies have deep interests in Syria, including the efforts to defeat IS, disrupt Iran-backed groups and contain the remnants of al-Qaida and other terror groups that have found sanctuary.

“I think it is a harbinger of more instability and the potential for more political violence,” Chris Costa, former senior director for counterterrorism in the first Trump administration, said last week before the fall of Damascus. Costa spent decades in the U.S. Army and ran special operations in combat zones. “I think there’s the potential for ISIS again to make trouble not just for the Iraqis but also be emboldened in the region.”

Alex Younger, who led Britain’s foreign intelligence agency, MI6, between 2014 and 2020, said a big concern is the “very large number of ISIS detainees left over from the destruction of the caliphate.”

Younger told the BBC that IS remnants are “currently contained by the Kurdish groups in the east, but if they go off the job, you can expect a serious spike in the threat posed to Europe by ISIS.”

Who will rule Syria?

Another problem is figuring out who’s in charge.

The opposition forces that stormed into Damascus and sent Assad fleeing to Russia are led by a group called Hayat Tahrir al-Sham, or HTS, that originally was part of al-Qaida, but split a number of years ago. HTS is considered a foreign terrorist organization by the U.S. and the United Nations.

Its leader Ahmad al-Sharaa, formerly known as Abu Mohammed al-Golani, has sought to reassure Syrians that the group is more moderate. U.S. officials say that while he may be saying some of the right things, they are adopting a wait-and-see attitude for now.

White House national security spokesman, John Kirby, told CNN News Central on Monday that while HTS “was the vanguard” they aren’t the only opposition group involved.

“We’re going to be working through all the processes we can, including at the U.N., to make sure that there is adequate communication with these opposition groups and that we are all working together,” he said.

Singh said she is not aware of any formal U.S. channel of communications with the group. But, she said, “We have other ways of getting messages through, you know, groups and other allies in the region.”

Associated Press reporters Eric Tucker and Sagar Meghani in Washington and Jill Lawless in London contributed to this report.