The United States needs to deploy airstrikes to stop the violence from escalating in Iraq, said James Jeffrey, the former U.S. ambassador to Iraq.
Rebels have already seized the key northern cities of Mosul and Tikrit and President Barack Obama is facing increasing pressure to assist Iraq.
For Jeffrey, there are three threats: if the rebels encircle Baghdad, if the Kurds say they don't want to be a part of Iraq and if Iran enters Iraq "in a big way."
"The only way to stop these three gamechangers for the entire Middle East … is for the Americans to come in in a big way, not on the ground but in the air," Jeffrey told CNBC's "Closing Bell."
When the president withdrew troops from Iraq in 2011, the country was at peace, he said. Now Iraq has allowed the situation to slip out of control.
The Iraqi administration and its supporters "weren't ready to make sacrifices for the common cause," said Jeffrey, who served as ambassador from 2010-2012.
In addition, Prime Minister Nouri al-Maliki and his government "almost went out of their way to alienate the Sunni Arabs," which is where the militant group is now finding support to some degree and a lack of resistance, he said.
Ilan Berman of the American Foreign Policy Council thinks the U.S. doesn't have many good options, particularly since the American electorate doesn't have the appetite for another Gulf War.
"The administration has a full-blown foreign policy crisis on its hands," Berman told "Closing Bell."
Iran has already sent elite troops to assist Iraq against the al-Qaeda-inspired rebels, according to the Wall Street Journal.
"We're beginning to see history repeat itself," Berman said. "Whether Iran wins or al-Qaeda wins, I think American interests are going to take a hit."
—By CNBC's Michelle Fox