Those American and Western citizens who have gone to Syria and are currently fighting or have fought for extremist/terrorist groups, like al-Nusra, ISIS, JMA, etc., will be trained in how to conduct both guerrilla warfare and terrorist attacks. Usually whenever intelligence brings them down, it is because the individual or cell communicates with who they thought was an al-Qaeda (using them as an example here) handler; in reality, said handler was an intelligence official. People have and will continue to slip through the cracks due to skill, luck and/or human error (like the failed Times Square Bombing, failed Christmas Day bombing, Richard Reid). In the case of these individuals fighting in Syria, they are skipping that crucial part of reaching out for guidance and assistance in how to conduct terrorist attacks; they already have that knowledge and could already have a plan concocted. More importantly, once these individuals come back to normal everyday life it becomes even harder to track them; more so if they fly below the radar. These Chechens, which are just a small fraction of experienced jihadist forces inside Syria mind you, pose a threat to us because they can and probably are training individuals in these tactics. We know that several European and American fighters are in the main Chechen group, Jaysh al-Muhajireen wal-Ansar (Army of the Emigrants and Helpers), which is a group that is composed entirely of foreigners. Like Risay said above, some Chechen fighters are mercenaries. This is true. However, those affiliated to the IK, to JMA, to JN, to ISIS, to Junud ash-Sham, etc., most certainly are not.