The Islamic Republic claims that it is fighting Islamic State (IS)—and has done so occasionally—but in reality it has benefited immensely from the terrorist group’s rise. The more chaos IS causes, the more opportunity Tehran has to further its quest for regional hegemony, as Ray Takeyh and Reuel Marc Gerecht write (free registration required):
Iran is using Islamic State’s ascendance in the Middle East to consolidate its power. The country is now the key ally keeping Iraq’s Shiites and the Alawite Bashar al-Assad regime standing against well-armed and tenacious Sunni jihadists. In those battles, Tehran will likely do just enough to make sure the Sunnis don’t conquer the Shiite portions of Iraq and Assad’s enclave in Syria, but no more. Meanwhile, . . . Tehran will strengthen its own radical Shiite militias.
The result could be a permanent destabilization of the Arab heartland. That would be a major victory for the Islamic Republic, which has seen its fortunes rise as Egypt and Turkey have become mired in crises and as Saudi Arabia, Iran’s one remaining serious Sunni rival, has gotten bogged down in a war in Yemen. . . .
If Sunni radicals were to establish terrorist cells inside Iran’s minority Sunni communities, Tehran might want to throttle IS. Barring that eventuality, however, the clerical regime has no interest in diminishing the sectarian bloodbath that has allowed it more influence than at any time since the 1979 revolution. At best, anti-Americanism brought Tehran fellow-travelers; IS, however, has given the regime its best shot at an empire. For Iran’s ruling elite, Shiite-hating Sunnis have never looked so good.
Read more on Foreign Affairs: https://www.foreignaffairs.com/articles/syria/2015-11-15/irans-isis-trap