The group that claimed credit for the deadly terrorist attack in Moscow on Friday is the Islamic State affiliate in Afghanistan called Islamic State Khorasan Province, or ISIS-K.

ISIS-K was founded in 2015 by disaffected members of the Pakistani Taliban, who then embraced a more violent version of Islam. The group saw its ranks cut roughly in half, to about 1,500 to 2,000 fighters, by 2021 from a combination of American airstrikes and Afghan commando raids that killed many of its leaders.

The group got a dramatic second wind soon after the Taliban toppled the Afghan government that year. During the U.S. military withdrawal from the country, ISIS-K carried out a suicide bombing at the international airport in Kabul in August 2021 that killed 13 U.S. troops and as many as 170 civilians.

The attack raised ISIS-K’s international profile, positioning it as a major threat to the Taliban’s ability to govern.

Since then, the Taliban have been fighting pitched battles against ISIS-K in Afghanistan. So far, the Taliban’s security services have prevented the group from seizing territory or recruiting large numbers of former Taliban fighters bored in peacetime — among the worst-case scenarios laid out after Afghanistan’s Western-backed government collapsed.

President Biden and his top commanders have said the United States would carry out “over-the-horizon” strikes from a base in the Persian Gulf against ISIS and Qaeda insurgents who threaten the United States and its interests overseas.