War is on the cusp of returning to northern Iraq. The commanders of Iran’s Islamic Revolutionary Guard Corps (IRGC) say preparations are being made for an all-out attack on Iranian Kurdish opposition groups based across the border in Iraqi Kurdistan. Unless Baghdad and Erbil disarm and move the groups away from the frontier by Sept. 19—under the terms of a bilateral security deal signed in March—an Iranian military offensive is impending. To this end, Iran’s ambassador to Iraq is engaged in a flurry of meetings while both Arab and Kurdish Iraqi officials have been hurrying to Tehran to defuse the situation. 

Kurdish armed groups, particularly the Kurdistan Democratic Party of Iran (KDPI) and the leftist Komala party, have been a threat to Tehran since the 1979 Islamic Revolution, when Kurds rebelled and fought a bloody war with the IRGC for nearly a decade. Some of the most prominent fighters and commanders of the Iranian elite force were killed in clashes with the KDPI and Komala Peshmerga fighters.

Despite the current contention between the two sides, the Peshmerga fighters and the political cadres of the Kurdish groups have...