It is generally acknowledged that the most efficient, ruthless and troublesome Islamic terrorists are the Chechens. This has been the case since the 1990s when Russia fought two wars in Chechnya to suppress separatist movements. Towards the end of the second war Russia provided Chechnya autonomy under the rule of a local warlord who was willing to keep Chechnya free of rebels and Islamic terrorists as long as his methods were not challenged. Because of that several thousand diehard separatists, many now Islamic terrorists, fled.
Chechens don’t like to fight Chechens, especially if the other side has Russia air support and special forces plus lots of Russian money. Many of those Chechen Islamic terrorists found work in places like Iraq, Syria, Pakistan, Afghanistan and Central Asia. The Chechens were effective terrorists but did not get along with other Islamic terrorists, especially Arabs. About 500 of these mercenary Chechens are still active, mainly in Syria and in Ukraine, where they received sanctuary only if they dropped the Islamic terrorism. Chechens are flexible and they did that for the Ukrainians. When Russia invaded Ukraine in 2014 the Chechens quickly joined and helped train the Sheikh Mansur battalion, which was composed of Ukrainian Moslems, mainly Crimean Tatars, who wanted Russia out of Crimea where the Tartars there were again being persecuted. Ukraine became independent of Russia in 1991 and the Crimean Tatars found life easier under Ukrainian rule. By 2015 Ukraine proclaimed the Sheikh Mansur battalion exceptional fighters and defenders of Ukraine. These volunteer units could be troublesome and were eventually replaced by army units. The Sheikh Mansur battalion disarmed in 2019 when asked to. But some members of the battalion, mainly Chechens, maintained a presence in eastern Ukraine where some thrived as gangsters. In 2020 Ukraine released a list of 557 career criminals in Ukraine and declared them enemies of the state and subject to expulsion if they were foreigners. Among those named were several Chechen gang leaders. Russia said it would be glad to take them back. Ukraine wanted to be rid of these Chechens but realized these fellows had fought Russians when Ukraine needed them, so sending them back to Russia might upset many Ukrainians who had fought, or are still fighting, the Russians in eastern Ukraine.
The remaining Chechen Islamic terrorists ended up in Syria where they joined the rebels and were effectively fighting ISIL (Islamic State in Iraq and the Levant), which was at war with the Syrian government as well as other Islamic terror groups. Most of the remaining Islamic terrorists in Syria ended up trapped in the northwest (Idlib province) where Chechens had their own group; Junud al Sham. As usual, the Chechens would not join the al Qaeda affiliate HTS that had become the main Islamic terrorist coalition in Syria and Idlib. HTS was able to negotiate with Syria and the Turks about a peaceful resolution for about 10,000 Islamic terrorists trapped in Idlib along with hundreds of thousand family members and other civilian supporters. Junud al Sham wasn’t just ab uncooperative group but they were the most dangerous and like their fellow Chechens in Ukraine a very difficult problem to deal with.
While only recent adopters of Islamic radicalism and terrorism, Chechens had been a problem for Russia since the 18th century. While the media likes to play up "terrorism in Chechnya," the main problem is that the Chechens, and their fellow Caucasians, have always been difficult to live near, much less control. This "Chechen Problem" has been on Russia's agenda since the 18th century, and nothing has really worked. Even Stalin deporting most of the Chechen population to Central Asia during World War II, when it was feared the approaching Nazis would find welcome allies among the Chechens, didn't fix the problem. This merely gave Chechens financial opportunities, usually criminal, throughout Russia. Chechens are still prominent among Russian organized crime organizations. In the 1950s, after the death of Stalin, the Chechens were allowed to return to Chechnya where they did not get along with the Russians, and others, who replaced them after the removal. Chechnya is not a new problem, it's an old one that won't go away.
Chechens have been fighting or fleeing Russia for two centuries, ever since Russia first took control of Chechnya. The fact that so many of these exiled Chechens are still be fighting in Syria is not surprising to most Russians, who are trying to identify Chechens in Syria who are Russian citizens and make sure that none of them get back into Russia. Most of the Chechens in Syria were not from Russia but volunteers from Chechen exile communities all over the world. No matter where they are, Chechens gotta Chechen, or at least some of them. Like many southern Italians drawn to the Mafia, Somalis to doing whatever it takes to get by and a handful similar groups, moving to a foreign country, often to get away from the bad history, takes generations to eliminate.
Meanwhile some countries take extreme measures. In 2012 Russia declared that it would longer conscript non-Russians from the Caucasus, especially Chechens. Since 2009 young men from the Caucasus had been complaining to the government that the army won't accept them as conscripts, or even as volunteers. The government insisted that those rejected had physical, psychological, or legal problems. But before long people it was revealed that the kids weren't being allowed in the military as an unofficial policy. Given the high unemployment rates in the Caucasus, and the warrior ethos common in many parts of the Caucasus, this ban from the military was becoming a major complaint. Instead of allowing young men from the Caucasus in at a time when there was a shortage of recruits, the government admitted it did indeed have an unofficial ban. The main (unofficial) reason was not wanting to train future terrorists, rebels, and criminals and to reduce disciplinary problems in the army.
While the army has been complaining of rampant draft dodging ever since the Soviet Union collapsed in 1991, they also have reasons for not wanting recruits from the Caucasus. Even before 1991, the largely ethnic Slav dominated Russian army warned company (units of about a hundred troops) commanders to not allow more than ten Chechens in their unit. Experience had shown that ten or more Chechens (or other men from the Caucasus) would form a very tight, tough, and disciplined clique that would prey on the other troops in the company and cause all manner of discipline and crime problems. If you find yourself with more than ten Chechens, try and transfer some of them out.
While the Chechens were the worst in this respect, the other Caucasus nationalities came close. After the 1990s the young men wanted to join the army and get a few years military experience, so they can qualify to become a "contract" soldier. These troops are paid a lot more and are considered "professional troops." Commanders actually prefer contract soldiers from the Caucasus, although many will admit that it's still not wise to have too many in an infantry company or platoon.