President Trump is threatening to launch missile strikes against Syria to punish President Bashar al-Assad’s government for a suspected April 7 chemical weapons attack on a Damascus suburb.

But Mr. Trump’s pick of targets is complicated by the presence of Russian troops and Iranian militias, who are supporting Mr. Assad’s forces in Syria’s seven-year civil war.

Where America, Russia and Iran Have Military Positions in Syria TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia GOVERNMENT CONTROL Deir al-Zour SYRIA Homs Palmyra Abu Kamal LEBANON Sparsely populated areas Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia Deir al-Zour GOVERNMENT CONTROL SYRIA Homs Palmyra Abu Kamal LEBANON Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Hasaka Afrin Manbij KURDISH CONTROL Idlib REBEL CONTROL ISIS CONTROL GOVERNMENT CONTROL Homs Abu Kamal Damascus IRAQ Sweida 100 MILES JORDAN TURKEY Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Russia Latakia GOVERNMENT CONTROL Deir al-Zour SYRIA Homs Palmyra Abu Kamal LEBANON Sparsely populated areas Damascus IRAQ Sweida JORDAN 50 MILES TURKEY Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Russia Latakia Deir al-Zour GOVERNMENT CONTROL SYRIA Homs Palmyra Abu Kamal LEBANON Damascus IRAQ Sweida JORDAN 50 MILES TURKEY Hasaka Afrin Manbij KURDISH CONTROL Idlib Russia REBEL CONTROL ISIS CONTROL GOVERNMENT CONTROL Homs Abu Kamal Damascus IRAQ Sweida 100 MILES JORDAN TURKEY Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia GOVERNMENT CONTROL Deir al-Zour SYRIA Homs Palmyra Abu Kamal LEBANON Sparsely populated areas Iran and

its proxies Damascus IRAQ Sweida JORDAN 50 MILES TURKEY Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia Deir al-Zour GOVERNMENT CONTROL SYRIA Homs Palmyra Abu Kamal Iran and

its proxies LEBANON Damascus IRAQ Sweida JORDAN 50 MILES TURKEY Hasaka Afrin Manbij KURDISH CONTROL Idlib REBEL CONTROL ISIS CONTROL GOVERNMENT CONTROL Homs Abu Kamal Iran and

its proxies Damascus IRAQ Sweida 100 MILES JORDAN TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Russia Latakia GOVERNMENT CONTROL Deir al-Zour SYRIA Homs Palmyra Abu Kamal LEBANON Sparsely populated areas Iran and

its proxies Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Russia Latakia Deir al-Zour GOVERNMENT CONTROL SYRIA Homs Palmyra Abu Kamal Iran and

its proxies LEBANON Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Hasaka Afrin Manbij KURDISH CONTROL Idlib Russia REBEL CONTROL ISIS CONTROL GOVERNMENT CONTROL Homs Abu Kamal Iran and

its proxies Damascus IRAQ Sweida 100 MILES JORDAN TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia GOVERNMENT CONTROL Deir al-Zour SYRIA Homs Palmyra Abu Kamal LEBANON Sparsely populated areas Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia Deir al-Zour GOVERNMENT CONTROL SYRIA Homs Palmyra Abu Kamal LEBANON Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Hasaka Afrin Manbij KURDISH CONTROL Idlib REBEL CONTROL ISIS CONTROL GOVERNMENT CONTROL Homs Abu Kamal Damascus IRAQ Sweida 100 MILES JORDAN TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia GOVERNMENT CONTROL Deir al-Zour SYRIA Homs Palmyra Abu Kamal LEBANON Sparsely populated areas Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Afrin Hasaka Manbij KURDISH CONTROL Idlib Raqqa ISIS CONTROL REBEL CONTROL Latakia Deir al-Zour GOVERNMENT CONTROL SYRIA Homs Palmyra Abu Kamal LEBANON Damascus IRAQ Sweida JORDAN 50 MILES TURKEY American-led

coalition Hasaka Afrin Manbij KURDISH CONTROL Idlib REBEL CONTROL ISIS CONTROL GOVERNMENT CONTROL Homs Abu Kamal Damascus IRAQ Sweida 100 MILES JORDAN

American forces in Syria have focused mainly on fighting the Islamic State in the country’s northeast.

More than 40 people were killed and dozens more sickened in the suspected attacks on the Damascus suburb of Douma over the weekend. Ever since, Mr. Trump has repeatedly promised to strike Syria — as he did last year following a deadly chemical attack in Idlib Province — and potentially pull the United States into the wider conflict.

American troops landed in the ground war in Syria in late 2015 with a small contingent of Special Operations forces, hoping to forge an alliance with local militias and rebel groups that could fight the Islamic State.

In the months that followed, the number of American troops grew. Their Kurdish and Arab allies, later known as the Syrian Democratic Forces, started ground assaults that would eventually lead to the loss of Islamic State strongholds in the northern cities of Manbij and their de facto capital in Raqqa.

There are currently an estimated 2,000 American troops in Syria, according to the Pentagon. The influx of forces transformed what had been an initial band of commandos in armored pickups into a scaled-down version of the sprawling military presence in neighboring Iraq.

They are spread across hundreds of miles of Syrian territory – contending with a potential Turkish offensive into Kurdish areas in Manbij, and into the middle Euphrates River Valley, where the remnants of the Islamic State are still holding out in the country’s east.

The Pentagon has said it is focused on eradicating the Islamic State extremists, and is fighting in areas that are uncomfortably near Syrian troops and Iranian forces. The United States military has had to deconflict airspace with Russian fighter jets in Syria. And American troops have crossed paths with Russian mercenaries and Iranian-backed militias, including a pitched battle in February and on the Iraqi border last spring. And in June, a United States military jet shot down a Syrian warplane that was trying to bomb American-backed fighters on the ground.

As a result, the combination of foreign actors, militants and local allies have vexed American commanders who are trying to keep their troops out of harm’s way without setting off an international confrontation.

U.S. special forces soldiers scan the area at a front line outpost outside the northern Syrian city of Manbij. Mauricio Lima for The New York Times

Beyond the contingent of Special Operations forces, the American presence in Syria includes conventional troops tasked with securing rural outposts, engineers for base construction, airmen for flight operations and trainers for the Syrian Democratic Forces, according to defense officials.

When American troops first arrived in Syria, their primary goal was ensuring that local forces fighting the Islamic State were well-trained, equipped and could attack with the necessary amount of air support backing them up. But as the campaign has crept on, American troops have needed an upgraded logistics system and a constellation of outposts to support the fighting.

In early 2016, an improved helicopter pad appeared in Syria’s northeast, near Hasaka. Months later, British Special Operations forces were pictured at a base near the border intersection of Syria, Jordan and Iraq farther to the south. The outpost, known as al-Tanf, would later turn into a key hub for American forces and allied Syrian rebels as they turned their assault toward the Euphrates River Valley.

That spring, American troops and Kurdish and Arab forces pushed toward the city of Manbij. Western forces — including a number of attack and transport helicopters — started appearing in satellite photos at a cement plant located between Manbij and Raqqa.

By the end of summer 2016, the American-led coalition and local forces had recaptured Manbij from the Islamic State, and the Pentagon had increased the number of United States troops in Syria to 500.