A force comprising thousands of Kurdish and Iraqi army soldiers wrested territory from the Islamic State outside the northern city of Mosul on Monday, facing occasionally fierce resistance at the start of a long-promised offensive to dislodge the extremists from their main stronghold in Iraq.

Kurdish forces moved to take a string of villages east of the captive city while Iraqi army and police units made a push from the south, a rare display of coordination and harmony between rival forces that officials hailed as a significant victory in itself. Kurdish officials said Monday evening that their forces had cleared nine villages in an area measuring roughly 75 square miles, although the degree of their control over the territory remained unclear.

Announced before dawn in a televised address by Iraqi Prime Minister Haidar al-Abadi, the battle is the most ambitious offensive launched by Iraq’s security forces since they were created after the U.S.-led invasion in 2003. As the sun rose and warplanes of the U.S.-led coalition circled overhead, giddy Kurdish soldiers known as peshmerga rode armored vehicles, land movers and even motorcycles on dirt roads toward front lines that seemed to advance by the hour. […]