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– The heavily armed fighters peered out a broken second-story window at their outpost in a crumbling house on the western edge of the city, where the Islamic State is fighting a furious battle to hold on to the capital of its self-declared caliphate.

They ducked to avoid snipers camped in nearby high-rise apartments. Armed drones hovered nearby. Just before 2 p.m. came the crack of sniper fire.

"That's our guy," Kevin Howard, 28, said as he rose and prepared to return fire.

Howard is not one of the hundreds of U.S. troops deployed in Syria. The U.S. Marine Corps veteran from San Francisco came here as a volunteer, part of a small group of freelance recruits who have traveled to Syria from the U.S., Europe and other regions to help local forces fight ISIS.

Americans have a history of volunteering to fight overseas. The Abraham Lincoln Brigade fought fascists in the Spanish Civil War, U.S. pilots flew for Britain and China before World War II, and U.S. citizens have served in the Israel Defense Forces.

The war in Syria and Iraq has been more problematic. Americans who try to travel there to fight alongside ISIS militants face immediate arrest, and many have been detained at U.S. airports as they prepared to respond to the terror group's global call to arms. Those who volunteer to fight the jihadis with U.S.-allied Kurdish and Syrian militias, though the State Department advises against it, face no such legal consequences.

Several hundred volunteers have arrived since the Syrian civil war began six years ago, according to local estimates, and several dozen remain.

Some, such as Howard, are military veterans who served in the Middle East and believed they had left a job unfinished. Others are young people drawn to the plight of the Kurdish rebels or by the powerful lure of combat in a faraway land.

In recent days, as the battle for Raqqa has turned into a violent death spiral for the ISIS fighters, three of the volunteers have died.

Nicholas Warden, 29, of Buffalo, N.Y.; Robert Grodt, 28, originally of Simi Valley; and Luke Rutter, 22, of Birkenhead, England, were killed as Kurdish forces, aided by coalition air support, advanced on Raqqa.

"Everyone is really torn up over losing those three guys, especially all at once. And they were only a few months out of the [Kurdish training] academy," said Lucas Chapman, who returned to Washington this year after fighting alongside the Kurdish People's Protection Units, or YPG, the largest force fighting in eastern Syria.

Warden had served with the U.S. Army in Afghanistan and with the French Foreign Legion, Chapman said, and came to fight the Islamic State in February because of the terrorist attacks in San Bernardino, Calif., and Orlando. Grodt graduated from Monte Vista School Independent Learning Academy in Simi Valley and studied philosophy at Moorpark College, friends and family said.

Howard had planned to leave once Raqqa was freed. So did Hudson. Now they are reconsidering.

"This is the most important fight in the world right now," Howard said.