As American jets flew top cover, in mid-August Kurdish Peshmerga militia and Iraqi special forces troops recaptured the strategic Mosul Dam from Islamic State militants. Meanwhile Iraqi Golden Brigade commandos liberated parts of Tikrit from the Islamists.

But the militants counterattacked—and that drew Iran into the fighting. In a move that could have far-reaching consequences, Tehran has sent tanks into northern Iraq.

As the Kurds and Iraqi commandos gained ground in the north, Islamic State fighters launched a surprise counterattack toward Baghdad. Local fighters resisted the militants north of Balad air base, formerly the center of the American occupational force. […]