Why would Islamic State attack Russia and what does this mean for the terrorism threat globally?

Over 130 people were killed in the attack on Crocus City Hall, a sprawling mall and concert venue on Moscow's western edge. Picture: Investigative Committee of Russia via AP

It appears almost certain the brutal assault on a Russian crowd settling down to watch a rock concert in Moscow on Friday night was an Islamist terrorist attack.

At least 133 people were left dead and scores more were injured after gunmen with automatic weapons stormed the Crocus City Hall in Moscow and opened fire, triggering a stampede.