Most constitutional law experts say Obama acted within his authority to defend Americans in Erbil from immediate danger. But critics say Obama should have sought congressional approval before expanding a fight with ISIS that could last months or even years.

“I think any conflict of a couple days in nature could be justified,” said Louis Fisher, a scholar at the Constitutional Law Project, “but as President Obama said last weekend, this is not going to be for just a couple days or weeks, it could go on for a year or two.

“For anything of that scope, he should come to Congress for their approval,” Fisher added.

Fischer and other constitutional scholars believe Obama is toeing a fine line by engaging ISIS forces in Iraq.

“Defensive power is limited to an immediate response to an attack,” said Peter Raven-Hansen, a national security professor at George Washington University’s law school.