NATO and American forces used a Tajik air base to gain access to Afghanistan from 2002 until last year, but now have left. Nevertheless, Afghanistan’s fate remains a major American interest, and so the West should push back against the rise of militancy and increase its aid to the region. In doing so, it will have to be willing to work with Russia, and vice versa.

Why not just leave Central Asia to Moscow to sort out, if it wants? One reason is that only a fast-flowing river separates the Central Asian states from Afghanistan, and Central Asian militants who have spent the past decade hiding out in Pakistan are now being helped by their Taliban allies in northern Afghanistan to get access to Central Asia. This year’s Taliban spring offensive has been the heaviest since 2001, with the militants’ main goal apparently to control strategic towns and territory along the entire 1,200-mile border with Central Asia.

I recently met with senior Tajik military and intelligence officers who rarely talk to reporters — and never do so on the record because they are forbidden to publicly discuss sensitive security issues. They told me that more than 5,000 Central Asian militants from half a dozen groups were fighting in northern Afghanistan alongside several thousand Afghan and Pakistani Taliban members. In late April, those forces came within two miles of Kunduz, a major Afghan city. The Afghan Army pushed them back, but only to 10 miles outside the town.

Other Central Asian militants are concentrated in the northeastern Afghan province of Badakhshan, a mountainous region adjacent to Tajikistan to the north, Pakistan to the south, and China to the east — a potential roundabout for multiple cross-border acts of terror.

Prominent among them is the Islamic Movement of Uzbekistan, made up of militants from a range of Turkic-speaking groups and bent on creating a caliphate from Turkey to Xinjiang Province in China, which is populated by Uighurs, a Muslim Turkic people. Tajik officials say other groups include the primarily Uzbek Jundullah; the Tajik-dominated Jamaat Ansarullah; and the East Turkestan Islamic Movement, largely made up of Uighurs. Kazakhs, Kyrgyz and Turkmen fighters are sprinkled through these groups.