ISIS, or the Islamic State of Iraq and Syria (Daesh) is not a novel phenomenon. It is a salafi-jihadist cult, propped up by the Saudi state and seasoned with US dollars. It achieves certain foreign policy objectives, and is largely out in the open in the modern day. Russia has previously fought off its predecessor - Al Qaeda. Both of the Chechen wars of the 1990s were funded and aided by Saudi Arabia and the US. NGOs such as Medicins Sans Frontiers were documented to have been smuggling supplies to the Chechen rebels, at the cost of hundreds of thousands of lives on both sides of the conflict.
On New Year's Eve 1999, Vladimir Putin would inherit the Russian presidency. With it, he inherited a very damaged and weary country. Given that the previous President, Boris Yeltsin, was in no physical state to rule - it is evident that Putin's work started long before his inauguration. In the 1990s and early 2000s, Russia was plagued by horrific acts of terrorism (Beslan, Nord-ost, etc), which in Western media were packaged as acts of "Chechen freedom fighters."
Should the rebels prevail, Putin envisaged the extensive loss of Russia's territories
and the possibility of an 'Islamic State'. Not only was the Russian government successful in their military campaign, despite a crumbling economy at the time, but they were able to mend ties with Chechen leaders, who today staunchly oppose radical Islam. With this understanding in mind, Putin's decision to engage in the 2015 Syrian campaign was completely logical. He knows where it leads and doing nothing is not an option.
Comment: It's a worrying sign of the insanity bubbling over within NATO when we see one of its top officials advocate greater confrontation with Russia when Moscow tries to extend an olive branch. There is absolutely no evidence of Russian interest in overtaking its neighbors, yet that doesn't stop the West from inventing ridiculous scenarios where Russia behaves like NATO actually does.