Obama right now relies on Bush's illegal "preemption," that is aggression in Iraq, for his precedent for going it alone in "humanitarian intervention"....This is confused, but the confusion is perhaps somewhat understandable because the Obama admin's statements on Syria have suggested several different, albeit related, rationales for a strike against Assad: (1) norm enforcement, (2) punishment/deterrence, (3) protection of the Syrian population from further chemical weapons attacks, and (4) prevention of chemical weapons possibly getting into 'the wrong hands' and being used against the U.S. or its allies. Only #4, which has not been emphasized that much, has any connection to Bush's 'preemption' doctrine (which was actually a prevention, not a preemption, doctrine). #3 is the humanitarian intervention rationale, which also brings in elements of #1 and #2.
The notion of humanitarian intervention has a very long, albeit controversial, history/pedigree in international law and practice, a fact that is apparently not widely understood. (It long predates Bush's preemption/prevention doctrine, which has nothing to do with humanitarian intervention.)
In her 2003 book The Purpose of Intervention: Changing Beliefs About the Use of Force, Martha Finnemore pointed out that "[b]efore the twentieth century virtually all instances of military intervention to protect people other than the intervenor's own nationals involved protection of Christians from the Ottoman Turks." (p.58) Over the course of the twentieth century the notion of who is 'human' and thus worthy of protection expanded to include non-Christians and non-whites. To quote Finnemore again:
...by the late twentieth century all human beings were treated as equally deserving in the international normative discourse. In fact, states are very sensitive to charges that they are "normatively backward" and still privately harbor distinctions. When Boutros-Ghali, shortly after becoming [UN] Secretary-General, charged that powerful states were attending to disasters in white, European Bosnia at the expense of non-white, African Somalia, the United States and other states became defensive, refocused attention, and ultimately launched a full-scale intervention in Somalia before acting in Bosnia. (p.83)Whether what the Obama admin is proposing to do w/r/t Syria is a good idea is debatable. But it's wrong to suggest, as a Democratic congressman did on the NewsHour last night, that the admin is seeking to create a "new category" of "humanitarian war."
P.s. (added later): Whether the notion of humanitarian intervention would have supported or required earlier, more forceful action by the admin w/r/t Syria is a legitimate question but in a sense irrelevant to the main point of this post.