The Islamic State (formerly ISIS) swept across northern Iraq over the weekend, reportedly defeating Kurdish forces for the first time, and was rumored to have captured a vital dam near Mosul.

First, the radical Sunni group was said to have taken control of three towns within the triangular border area between Iraq, Syria, and Turkey. Previously held by the Kurds, this corridor will presumably allow the group to pass weapons and fighters between the conflicts in Syria and Iraq.

In addition to the dozens reported killed, there is also a burgeoning humanitarian crisis created by the offensive. From the Times:

The United Nations representative in Baghdad, Nickolay Mladenov, issued a statement on Sunday afternoon, citing reports he had that as many as 200,000 civilians, mostly from the minority Yazidi community, had fled the new fighting.

Then, there was the matter of the Mosul Dam. From CNN:

ISIS took control of Iraq's largest hydroelectric dam on Iraq's Tigris River, which provides power to the city of Mosul about 50 kilometers (31 miles) to the south, the commander of the Peshmerga Kurdish fighters who had been defending the facility said Sunday.

The dam workers remained inside the facility, which fell after a 24-hour battle, Lt. Col. Herash said.

Throughout the day there had been alternating assessments about whether the Islamic State forces had actually captured the dam.

But what was not the subject of conjecture was the strategic importance of the Mosul Dam. As Tim Arango explained:

Keeping the dam, and other important infrastructure of the Iraqi state, out of militant hands has been a priority of the Iraqi government and the American military advisers who recently rushed back to Iraq. The loss of the dam would be a significant blow to efforts to contain the growing crisis.

According to a report by the Office of the Special Inspector General for Iraq Reconstruction, if the dam were to fail, it could unleash a 65-foot wave across northern Iraq and potentially flood and kill many in several surrounds towns and villages.

The Kurds have recently requested military assistance from the United States in battling ISIS, which places American diplomats in a precarious place as they try to keep intact the Iraqi state as we know it. Giving the Kurds weapons would upset the balance trying to be struck between the Sunnis, Kurds, and Shiites in Iraq.