GREATEST WAR EVER:

The large number of "foreign fighters" from Russia and the Central Asian states who have joined ISIS compounds the ISIS threat to Russia.

Last year, Russian President Vladimir Putin estimated the number of fighters who had left for Syria and Iraq from Russia and the former Soviet republics at 5,000 to 7,000.

The Soufan Group, a New York-based intelligence consulting firm that tracks foreign fighters who have joined ISIS, placed the number of fighters from Kyrgyzstan and Uzbekistan, another post-Soviet central Asian state, at 500 each.

ISIS has also relied upon its Central Asian recruits to carry out attacks in the past. ISIS recruits from Kyrgyzstan, Uzbekistan and Russia killed more than 40 people at Istanbul airport in June 2016.

If the attack on St. Petersburg was carried out by ISIS relying upon at least one Central Asian recruit, it would provide further evidence of the expansive and multi-layered threat ISIS poses to Russia.

ISIS despises the Russian government for its support of the Syrian dictator Bashar al-Assad, and so it's no surprise that ISIS began targeting Russia in 2015, around the same time that Russia first intervened in the Syrian civil war.

On October 31, 2015, ISIS bombed a Russian airliner carrying vacationing passengers from Sinai, Egypt to St. Petersburg, killing 224 people. ISIS celebrated the attack both in its English language magazine Dabiq as well as in its Russian language magazine Istok. The attack carried out by ISIS' Sinai affiliate illustrates how wide-ranging the threat to Russia is.

As ISIS loses on the battlefields of Iraq and Syria, contingents of Russian ISIS fighters who survive may try and make their way home to foment additional terrorism on Russian soil.

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