Bangladesh, Jan. 13 -- The renewed activity of Al Qaeda–affiliated groups in Syria has revived an old but unresolved question in Middle Eastern politics: how did an extremist Sunni jihadist organization and a revolutionary Shiite state come to intersect operationally in the same theaters of conflict? At first glance, the idea appears implausible. Al-Qaedas ideology is violently hostile to Shiism, while Iran has positioned itself as the protector of Shiite communities and the spearhead of resistance against Sunni extremism. Yet history repeatedly demonstrates that ideology often yields to strategy, and that adversaries can become functional partners when interests converge.

To understand this convergence, one must revisit the origin...