Russia’s Battle With Extremists Has Simmered for Years

In the past few months, deep in the forbidding deserts of central Syria, Russian forces have quietly joined the Syrian military in intensifying attacks against Islamic State strongholds, including bombing what local news reports called the dens and caves where the extremist fighters hide.

While the world was focused on the conflicts in Ukraine and Gaza, this type of skirmishing has been simmering for years in Syria, and the Islamic State has long threatened to strike Russia directly for shoring up the regime of its sworn enemy, President Bashar al-Assad of Syria.

That moment appeared to have come on Friday night with the bloody assault on a Moscow concert hall that left more than 130 people dead. “The fiercest in years,” said a statement of responsibility issued on Saturday by a branch of the Islamic State via its news agency, referring to the long history of brutal terrorist attacks pitting jihadist forces against Moscow.

“They have framed this attack as coming in the context of the normal, ongoing war between ISIS and the anti-Islamic countries,” said Hanna Notte, a Berlin-based expert on Russian foreign and security policy at the James Martin Center for Nonproliferation Studies. “This seems to be within the overarching theme of Russia in Afghanistan, Russia in Chechnya, Russia in Syria.”

In his brief remarks on Saturday, President Vladimir V. Putin of Russia did not mention the claim from the Islamic State, but he did threaten to punish those responsible. “All perpetrators, organizers and commissioners of this crime will receive a just and inevitable punishment,” Mr. Putin said.