"As a matter of domestic law, legal authority for the use of military force against Daesh and al-Qa'ida includes the Authorizations for Use of Military Force (AUMF) of 2001 and 2002," Assistant Secretary for Legislative Affairs Mary Waters wrote in a letter to Senator Tim Kaine.
In her letter, Waters argued that international law also provides a legal basis for keeping US forces in Syria to protect Iraq and the United States from terrorists.
The two letters came in response to Kaine's request that the Pentagon and State Department provide more information about the counter-Daesh mission in Iraq and Syria and clarify its objectives.
In his own correspondence, Kaine expressed his concern that the United States would soon find itself lacking legal justification for the wars in Iraq and Syria, particularly if US-led forces began combating the Syrian government, Iran, Iranian proxies or other groups not mentioned or alluded to in the 2001 AUMF.
"The United States does not seek to fight the Government of Syria or Iran or Iranian-supported groups in Iraq or Syria," Waters said in her letter. "However, the United States will not hesitate to use necessary and proportionate force to defend US, Coalition, or partner forces engaged in operations to defeat Daesh and degrade al-Qa'ida.
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The AUMF, first passed by Congress on September 14, 2001, gave then US President George W. Bush the power to use military force against al-Qaeda and any associated forces responsible for September 11, 2001, attacks on the United States.
Nearly 2,000 US troops are currently operating in Syria, though the Syrian government has not authorized their presence in the country.