That notification said the new missions would be “limited in their scope and duration as necessary to protect American personnel in Iraq by stopping the current advance on Erbil by the terrorist group Islamic State of Iraq and the Levant and to help forces in Iraq as they fight to break the siege of Mount Sinjar and protect the civilians trapped there.”
Obama needed to inform Congress of the air strikes because the legal authority for the president’s new Iraq war stems from the U.S. Constitution’s Article II, which has been interpreted by modern presidents to allow the president to order military action abroad without the consent of Congress.
The 1973 War Powers Resolution requires presidents to notify Congress when invoking Article II powers and to seek authorization from Congress if the new conflict lasts more than 60 days.
In this sense Obama’s new Iraq war is for now undeclared, even though the authorization for the old Iraq war remains on the books. Obama campaigned in 2012 in part on his accomplishment of ending the Iraq War and as recently as last month, his administration urged Congress to repeal the 2002 law that authorized it.