On December 13th, 2018, the Senate voted to end US involvement in the Saudi-led war in Yemen. This NPR article states, "the House would have to pass the resolution by year's end and President Trump would have to sign it." I searched and found the actual text of the resolution (Senate Joint resolution 54). The first line states, "To direct the removal of United States Armed Forces from hostilities in the Republic of Yemen that have not been authorized by Congress."
Requiring the President to sign the resolution means he effectively makes the final decision on the matter (by signing or not signing), even though from a constitutional perspective (at least through the lens of the War Powers act), Congress is supposed to make the final decision about whether to be at war through a simple majority vote (not a veto override).
There's two parts to my question. Firstly, if the hostilities weren't authorized in the first place (making them unlawful under the War Powers Act), why does Congress need the President's permission? It seems backwards to me.
Secondly, I looked at the text of the War Power act, (50 U.S.C. 1544(c)), and it states,
Notwithstanding subsection (b), 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.
This is the provision cited in Senate Joint Resolution 54. And that's precisely the issue. Why is this a joint resolution (which requires the President's signature) when the War Powers act only requires a concurrent resolution (which doesn't require the President's signature)? Is this a mistake by the sponsors of the resolution? Is it a way to avoid constitutional issues?