A look at the domestic legal justifications the U.S. has used for military action around the world:
IRAQ AND SYRIA
Congress passed an Authorization for the Use of Military Force in Iraq in 2002, clearing the way for the U.S.-led war in Iraq. Obama is taking action in Syria, acting under the same use-of-force authorization. That authorization still is on the books but President Barack Obama has called for it to be repealed but he has also used it as support for strikes against terror targets in Yemen and Somalia..
The White House says Obama had the power to authorize the airstrikes currently underway in Iraq through the inherent commander-in-chief powers outlined in the U.S. Constitution and it has made a similar argument regarding the airstrikes in Syria. Officials note that Iraq's sovereign government requested U.S. military assistance to combat Islamic State militants.
In the days after the Sept. 11, 2001, terror attacks, Congress passed an Authorization for the Use of Military Force that was meant to give President George W. Bush the ability to go after al-Qaida in Afghanistan.
PAKISTAN, YEMEN and SOMALIA
The Bush and Obama administrations have both used the 2001 force authorization to justify drone strikes against terror targets in Pakistan, Yemen and Somalia. The U.S. also has back-channel consent for the strikes from Pakistan's government and a more overt agreement with Yemen.
Obama did not seek congressional approval when the U.S. joined NATO allies in a 2011 bombing campaign in Libya. Officials argued that the nature of the mission, which did not involve American combat troops and was commanded by NATO, did not require authorization from lawmakers.