The Obama administration has decided to stretch the 15-year old congressional authorization for war against the perpetrators of the 9/11 attacks, or those harboring them, to include an illegal war against a group in Somalia--al-Shabab--that wasn't even in existence at the time of the attacks in 2001.
In fact, as with many of its Islamist terrorist opponents worldwide--including the original al Qaeda, the perpetrator of 9/11 that arose from U.S. arming of Mujahideen guerrillas against the Soviet Union in Afghanistan in the 1980s and al Qaeda in Iraq, which arose to combat the U.S. invasion there and morphed into ISIS--the United States inadvertently helped create al-Shabab in the first place. Al-Shabab did not arise until after 2007, long after 9/11, when the U.S. sponsored an Ethiopian invasion of Somalia to wrest control of the country from a milder Islamist council. The more virulent al-Shabab was rose to attempt to repel this foreign invasion.
More generally, after 9/11, rather than following the congressional authorization and focusing like a laser beam on countering the original al Qaeda group and their patrons, the Afghan Taliban, the George W. Bush administration launched a general "war on terror," which covered all terrorist groups of international scope, regardless of whether or not they focused on attacking U.S. targets. In the end, this massive Bush administration violation of the narrow 2001 authorization led to illegal U.S. drone wars and airstrikes in countries all over the Middle East and Southwest Asia--Somalia (against al Shabab), Yemen (against al Qaeda in the Arabian Peninsula), Pakistan (against the Pakistani Taliban), and Iraq, Syria, and Libya (against ISIS). The Obama administration then accelerated all these unconstitutional wars. Now Obama is trying shore up the already thin legal fig leaf, so that it can pass such travesties--which actually make Islamist groups more rabid each time the U.S. intervenes--onto the incoming Trump administration. When Obama took office, he complained that he inherited from the Bush administration an economic meltdown and a military quagmire in Iraq, but he in turn is bequeathing a legal quagmire to his successor.
Ambiguities in the U.S. Constitution do exist, but which branch of government was given the war power is not one of them. In 18th century Britain, the prerogative of deciding to go to war was the king's. Having been a victim of this prerogative, debates at the American Constitutional Convention of 1787, the Constitution that resulted, and actual practice in the republic for almost two centuries until the Korean War in 1950 demonstrate conclusively that Congress--the people's branch--gets to initiate war, not the executive. The Constitution specifically gives Congress the power to commence war; the debates at the Constitutional Convention indicate that the only exception is for urgent self-defense--that is, when U.S. territory is under sustained attack, thus preventing the Congress from convening. Even then, the Congress should meet at the earliest possible time to ratify any moves in self-defense made by the president, as commander in chief. Very early in American history, even in the informal and sporadic war at sea with France (the Quasi-War) in the last few years of the 18th century, the Congress was in the driver's seat in conducting the war and President John Adams complied with its desires. And in contrast to presidential claims of an expansive commander-in-chief role since the Korean War, the Constitution's framers intended, and normal practice till 1950 confirmed, that the president's role in that capacity was taken narrowly to mean only commanding troops on the battlefield after war had already been initiated by Congress--not commanding the entire nation, in times of crises or otherwise. Yet since 1950, presidents have claimed powers to start wars even without any authorization from Congress--either getting none (for example, Bill Clinton in his war to separate Kosovo from Serbia in 1999 or Barack Obama in overthrowing Muammar Gaddafi in Libya in 2011) or claiming that they needed to do so only as a courtesy, which was primarily a gambit to win increased political support for their military escapades (for example, both Bushes in each of their misadventures in Iraq).
Another trick is what Bush and Obama have done with the aforementioned drone wars--trying to blatantly fold wars against other only tangentially-related "Islamist" groups in countries far from Afghanistan into the congressional authorization for war against the perpetrators of 9/11--the original al-Qaeda group and their hosts, the Afghan Taliban.
Such legal gymnastics must stop. During the Trump administration, the many drone wars either must be made legally legitimate, with specific approval for each of them from the people's houses of Congress, or they must be stopped. The latter solution would be preferred--because those counterproductive foibles are making the threat from Islamist terrorism more virulent with each U.S. military intervention--but even the former option would at least put the wars on a much sounder constitutional footing.