Even if all these criteria were met -- even if Cameron had been defending a well-deserved, last-ditch military campaign for the right reasons using appropriate means and with the best possible plan to sustainably mitigate rather than increase civilian bloodshed -- it would still violate the R2P doctrine if it included the right to act unilaterally. Precisely because the humanitarian intervention norm runs afoul of the U.N. Charter and because fears are so great that it could be used as a smoke screen for wars of aggression, international support for this emerging norm has always been predicated on the idea that it would be used only where a broad multilateral consensus existed that it is the right thing to do.
Consider state practice since 1990. The "good wars" perceived by the international community to have been legitimate cases of humanitarian intervention include operations in Somalia to protect food shipments, northern Iraq to protect Kurdish refugees from attack, Bosnia and Kosovo to end ethnic cleansing, and Libya to forestall a devastating siege. In each case, these efforts were undertaken by a wide coalition of governments deeply invested in the cause. Now consider cases where a single government asserted the right to act unilaterally for humanitarian reasons. When the Russian military entered Georgia in 2008, it claimed it was doing so to protect civilians. When the United States invaded Iraq in 2003 to "liberate its people from a tyrant," the vast majority of states opposed this action as an ill-conceived violation of the U.N. Charter. Neither diplomats at the time nor analysts of political history include these incursions among the canon of legitimate humanitarian interventions. Whatever complex mixture of motives underlay these wars (and whatever mixture underlies "bona fide" interventions like Kosovo and Libya), it is multilateralism that constitutes a perception that a military intervention is legitimately humanitarian.
This importance of collective action is reflected in the codified R2P principles as well. Although lacking the status of treaty law, the R2P doctrine has been laid out in several international documents. In each one, it states the importance of seeking prior U.N. Security Council authorization for action, subsequently calling emergency sessions of the General Assembly, and coordinating emergency operations through regional organizations on a multilateral basis. The 2005 World Summit Outcome document referred to governments' collective "responsibility" to prevent atrocity. These documents do not confer a right upon individual states to decide for themselves. This explains why, despite a willingness to act without a Security Council authorization, the United States is bending over backward in its political rhetoric to emphasize the shared condemnation of Syria by allies in the region and around the globe. And it is particularly important as the United States gauges how to proceed with the military support of an important ally now out of the picture.
Why is multilateralism so important? It is partly because R2P represents a deeply cautious and tentative compromise between two important sets of rules -- the primacy of sovereignty and the primacy of human rights -- each of which plays a vitally important role in promoting human and global security. Neither should be easily disregarded on a whim by a single actor. While human rights must be protected, the U.N. Charter system itself is a collective public good: It ended a bloody history of great-power war and has ushered in the longest era of interstate peace in human history. States rightly allow exceptions to these fundamental rules only in extreme cases.
The rule that governments should collectively judge whether that threshold has been met is also a check on hubris. R2P channels collective outrage, but when push comes to shove collective reticence is often a canary in a coal mine. That key members of the international community -- including countries like Turkey that have a valid self-defense argument due to refugee flows, and members of the Arab League that would be happy to see Assad gone -- are unready to themselves take the lead would be viewed through an R2P lens as an indication that caution and deliberation is warranted. Does this mean inaction is the best policy? Maybe. Maybe not. But it does mean that unilateral intervention, even to ostensibly protect civilians, doesn't make a war "humanitarian" in the court of global public opinion.