Kurdish forces dig in just outside ISIS headquarters in Raqqa

When Bahoz heard the blasts, he guessed they must have come from French jets. There were 14 of them, all around the time that President Francois Hollande said France had started bombing Raqqa.

The capital of ISIS's self-declared caliphate is eerily close to Bahoz's position. A fighter with the Kurdish YPG units, he sits on a series of outposts along a lengthy earth trench that is essentially the front line with Raqqa -- about 20 miles away, across flat, hostile ground.

"Three days ago we saw 14 airstrikes suddenly hit just nearby, and then the French said they'd started bombing," he told CNN, when we were given rare access to his position near the town of Ayn al Issa.

"We will do our best to avenge Paris," he vowed.

Raqqa is now firmly in the sights not only of the U.S.-led coalition, but also the French and Russian militaries. And in a few hours along the front line, you can periodically hear distant thuds.

On the day CNN was at the front, they could have come from some of the four Russian missiles activists' group Raqqa is Being Slaughtered Silently reported hit the town, or the four homemade Katyusha rockets they also reported were fired by ISIS.

The Kurdish fighters here are the defensive line put in place in case a ground invasion begins; there are anti-ISIS militia massing, CNN has repeatedly been told by Kurdish officials with the YPG fighting units, but the operation has yet to begin earnest.

So for now these young men, who say they consider fighting ISIS a duty for humanity rather than a task of vengeance for the friends they have lost, are on the front line in a global battle.

And they have very little in the way of weaponry. Mostly old AK-47s; one fighter told us his used to belong to a friend who died eight months ago