The man who killed Usama Bin Laden, retired Navy SEAL Rob O'Neill, joined Megyn on "The Kelly File" tonight to discuss the beheading of American hostage Peter Kassig at the hands of ISIS and the threat that terrorists pose to the U.S.

O'Neill explained that ISIS is largely made up of the same combatants we were fighting during the War in Iraq, when we called them Al Qaeda in Iraq.

"It's the same group of guys, but once they knew we were going to leave, they backed off and waited. They knew that they could rise up and start fighting their other enemies, the Shia," he said.

He added that both the U.S. and ISIS know that we could defeat them in a heartbeat, but the terror group also knows that we are unsure of the American people's will to go back into the Middle East and fight with boots on the ground.

Watch more above to hear O'Neill discuss the blowback he's felt for speaking out about the mission that killed Usama Bin Laden.