Bombing ISIS is a continuation of an existing doubly authorized conflict.
I spent the last few years arguing that Obama's unilateral regime change operation in Libya was illegal and that his plans to bomb Syria without Congressional approval were illegal.
But the reasons weren't territorial.
Bombing Syria would have meant a new war against a new enemy and a nation state. That's not the case when the US is hitting ISIS, as it did when it attempted to rescue James Foley.
The original September 11 authorization covered the "use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons."
Some say that's overly broad, but ISIS is covered under both past and future.
It was aligned with Al Qaeda originally. Its relationship with Al Qaeda is more complicated now, but it's very much a part of the Jihadist network. And it has killed numerous Americans over the years and has threatened attacks against America.
The US has been involved in conflicts with ISIS, in its various forms, for nearly a decade.
No new authorization was needed when the US bombed Al Qaeda targets in Yemen or when it went deep into Pakistan after Bin Laden.
The problem is that Obama has yet to announce that he's relying on the original AUMF or even the Iraqi one because he's pathologically uncomfortable with both wars. That's his problem. His legal grounds for bombing ISIS are therefore unnecessarily shaky, but the bottom line is that the default authorization is still there.
Finally fighting terrorists is not really a war in the conventional sense. What we're currently doing is more like cross-border raids against insurgents that have more in common with the pursuit of Pancho Villa than WW1. There's plenty of precedent for engaging in such low scale conflicts without Congressional authorization as these are not really wars with nations.
It's still a good idea for Congress to approve military action and pass legislation necessary to address any specific issues, but ISIS is fairly special case as it's a continuation of an existing doubly authorized conflict.