Hunkered down on the top floor of an abandoned building, two Americans and a British volunteer face off against Islamic State snipers in the Syrian city of Raqqa. The trio, including two who served in the French Foreign Legion and the war in Iraq, have made the war against IS their own.

They are among dozens of Western volunteers who have battled the Islamic State group in Iraq, and now in Raqqa, the city in northeastern Syria that the militants declared the capital of their self-proclaimed caliphate.

The men joined U.S.-allied Syrian militias for different reasons — some motivated by survivors' accounts of brutality at the hands of the extremists. Others joined what they see as a quest for justice and a final battle to tear out the "heart of darkness."

Taylor Hudson, a 33-year old from Pasadena, California, compares the fight for Raqqa to the 1945 Battle of Berlin in World War II that ended the rule of Adolf Hitler.

"This is the Berlin of our times," said Hudson, who doubles as a platoon medic and a sniper in the battle against the militants. For him, IS extremists "represent everything that is wrong with humanity."

Syria's war, now in its seventh year, has attracted foreign fighters to all sides.

Extremists from Europe, Asia and North Africa have flocked to IS as well as local al-Qaida-linked groups. Shiite Iranian and Lebanese militias have sided with the Syrian government, deepening the sectarian nature of the conflict that has killed over 400,000 people and displaced over 11 million, half of Syria's pre-war population.

A much smaller number of Western volunteers fight alongside the U.S.-allied Kurdish militia known as the People's Protection Units, or YPG. The U.S. military has developed a close relationship with the YPG and its extension, the Syrian Democratic Forces, in the war against IS.

Before that, dozens of Westerners joined Iraqi Kurds fighting IS, spurred on by Kurdish social media campaigners and a sense of duty many feel after Iraq, the target of a decade-long U.S.-led military campaign, collapsed under an IS offensive within days in the summer of 2014.

Some Western volunteers have died in battle. Earlier this month, the YPG announced that 28-year-old Robert Grodt, of Santa Cruz, California, and 29-year-old Nicholas Alan Warden, of Buffalo, New York, died in the battle for Raqqa.

Since launching the push on Raqqa on June 6, the U.S.-backed forces have taken a third of the city.

Hudson, who has been fighting in Syria for the past 13 months, said he was moved to tears by media reports of Iraqi Yazidi women enslaved by IS. A pharmacy student who learned combat medicine in the field, he said he had treated some 600 wounded ahead of the march onto Raqqa.

The presence of Western anti-IS volunteers in Syria has created something of a conundrum for their governments, which have often questioned them on terrorism charges.

"I am not a terrorist," said Macer Gifford, a 30-year former broker in London, who came to Syria three years ago to volunteer first with the Kurdish militia. Now he is fighting with an Assyrian militia, also part of the U.S-backed forces battling IS.

"I am here defending the people of Syria against terrorists," he added.

Gifford has been questioned by British and U.S officials. At home, he has written and lectured about the complex situation in Syria, offering a firsthand account of IS' evolving tactics.

"The Islamic State is actually an exceptional opponent," Gifford said. "We can't negotiate them away, we can't wish them away. The only way we can defeat them is with force of arms."

For Kevin Howard, a 28-year old former U.S. military contractor from California who fought in Iraq in 2006, the war is more personal.

A skilled sniper who boasts of having killed 12 IS militants so far, Howard said he is doing it for the victims of the Bataclan theatre attack in France, where the sister of one of his best friends survived. The Nov. 13, 2015 attacks, claimed by IS, killed 130 people at Paris cafes, the national stadium and the Bataclan, where 90 died.

"This is a continuation of that fight. I think if you leave something unfinished, it will remain unfinished for a lifetime," he said, showing off his 1972 sniper rifle.

On his forehead and neck, he has tattooed "life is pain," as well as "Rien N'empêche" — or "Nothing Prevents"— from the song of the French Foreign Legion in which he served.

"For me this is a chance to absolutely go to the heart of darkness and grab it and get rid of it," he added.

From his sniper position on Raqqa's front line, he peeked again through the rifle hole. For Howard, the orders to march deeper into the IS-held city can't come soon enough.