The sudden rout of Iraqi security forces in Mosul, Iraq’s second-largest city, is a humiliating defeat for the Iraqi government, a severe blow to U.S. policy in Iraq, and a strategic disaster that will amplify the threat posed by al-Qaeda-linked terrorists to the United States and its allies.

The swift victory of the Islamic State in Iraq and Syria (ISIS), formerly known as al-Qaeda in Iraq, demonstrates the growing threat posed by Islamist militants in the region and the risks inherent in the Obama Administration’s failure to maintain a residual U.S. military training and counterterrorism presence after the withdrawal of U.S. troops in 2011.

Iraqi security forces collapsed and retreated from Mosul in the face of ISIS militants recruited from Iraq, Syria, and foreign Sunni extremist movements. The defeat underscored the weakness of Iraq’s armed forces, which was apparent long before the U.S. withdrawal.

The insurgents not only captured significant amounts of arms and equipment abandoned by the demoralized security forces; they also seized about 500 billion Iraqi dinars (approximately $429 million) from Mosul’s central bank. This will make ISIS the richest terrorist group ever and enable it to further expand its power by buying the support of Sunni Iraqis disenchanted with the sectarian policies of Prime Minister Nouri al-Maliki’s Shia-dominated government.

Although the resurgence of ISIS has been enabled by Maliki’s heavy-handed rule and the spillover of the increasingly sectarian civil war in Syria, the Obama Administration also played a counterproductive role in downplaying the prospects for an al-Qaeda comeback in Iraq.

The Administration early on made it clear to Iraqis that it was more interested in “ending” rather than winning the war against al-Qaeda in Iraq. As Heritage Foundation analysts repeatedly warned, the abrupt U.S. troop withdrawal in 2011 deprived the Iraqi government of important counterterrorism, intelligence, and training capabilities that were needed to keep the pressure on al-Qaeda and allowed it to regain strength in a much more permissive environment.

Now ISIS, whose leader in 2012 threatened to attack the “heart” of America, poses a rising threat to U.S. security. The bottom line is that the Obama Administration’s rush to “end” the war in Iraq has helped create the conditions for losing the war against al-Qaeda.