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The Founders of the US Constitution intended that the power to wage war reside directly under the authority of the peoples’ representatives in Congress. The War Powers Resolution of 1973 (also known as the War Powers Act) has several provisions, but we’ll focus on this part in the section on Congressional Action:
"…at any time that United States Armed Forces are engaged in hostilities outside the territory of the United States, its possessions and territories without a declaration of war or specific statutory authorization, such forces shall be removed by the President if the Congress so directs by concurrent resolution."
A concurrent resolution of Congress requires a majority vote of the members of the Senate and the House.
Another way for Congress to end the wars is to refuse its funding.
Either path would lead to an expeditious safe withdraw of US troops (no one is advocating for an exit that is dangerous to our troops). If Afghanistan feels they need security assistance, they are welcome to ask the UN Security Council.
An important point in evaluating the current wars is that the US invasion was unlawful. Click here for the full explanation of relevant laws. The short version: the UN Charter makes all war unlawful that is not in self-defense to stop an armed attack from another nation and/or authorized by the UN Security Council. The US attacked and invaded Afghanistan four weeks after 9/11, which fails the legal definition of self-defense. The attack was in violation of UN Security Council Resolution 1373, which resolved for international cooperation under the law.
Representative Dennis Kucinich makes his argument of why the US should end war in Afghanistan in the following 10-minute interview with Amy Goodman.