An old saw has it that "Arab unity" is an oxymoron on par with "military intelligence." Read not as racial essentialism but as a critique of pan-Arabism, the observation has been true in the modern era. Yet Egyptian President Abdel-Fattah El-Sisi's announcement of an agreement "in principle" by the Arab League to create a joint military force may just be different. Because the Islamic State group is unlikely to be defeated by air power alone, the United States should probably welcome the step — as should Israel.

The specific politics that generated the proposal have to do with Egypt's desire to re-establish its stature in the Arab world post-Arab Spring, and with Saudi Arabia's desire to keep an Iranian-backed regime from gaining a foothold in Yemen.

Start with Egypt's interests. It was no coincidence that the announcement coming out of the Arab League summit in Sharm el-Sheikh was made by Sisi. It's not just that he was the summit's host. Foreign policy is part of Sisi's effort to solidify his domestic legitimacy after the Egyptian Army's coup against the elected government of Mohamed Mursi.