Story highlights At least 10 people died in a blast on the St. Petersburg metro

Russian authorities say the explosion is a terrorist attack

(CNN) When President Vladimir Putin first ordered Russia's air campaign in Syria in late 2015, his mission was clear: to kill the terrorists living there before they were able to kill people in Russia.

Thousands of people from the Russian Federation, mostly from the Caucasus region, had traveled to Syria to join Islamist groups . Many have developed reputations for being fierce, capable fighters.

The Russian government, like those in France, Belgium and the UK, have long feared that its own battle-hardened citizens could return to wreak havoc.

While Russia's role in the Syrian civil war has been interpreted by some in the West as a cynical and self-interested move to prop up the Assad regime -- US President Barack Obama described Putin's involvement in Syria as being "not out of strength, but out of weakness because his client Mr. Assad was crumbling" -- it was sold to the Russian people as a necessary counterterrorism operation.

Some analysts believe that while Russia's activity in Syria has successfully killed many potential terrorists, it may also have motivated many others to strike back at Russia.