President Obama’s speech last evening offers a chance to assess the implications of the war in Libya.

President Obama is not the first president to order attacks on another nation without the authorization of Congress. This case, however, seems different. Prior to the intervention, the President’s national security advisors had determined that the nation had no vital interest at stake in the Libyan civil war. Secretary of Defense Robert Gates has repeated that conclusion after the intervention began. For his part, President Obama emphasized in last night’s speech and before, that the war would preclude a “humanitarian catastrophe.” Why did that rationale win out over the realism of his advisors?

President Obama tends to see our nation and the world as divided between oppressors (victimizers) and the oppressed (victims). In this view, politics should help the oppressed and do justice (i.e. harm) to the oppressor. In Libya, this outlook provides a clear division between a oppressor (Qaddafi and his loyalists) and his victims (the rebels). Morality thus demands war against the oppressor on behalf of his victims.

But there is a problem with America acting alone. Many people in the Middle East and elsewhere see the United States not as a vindicator of the oppressed but rather as a oppressor. Truth be told, more than few Americans share that view.

Those who share this view believe that the United States cannot act unilaterally to help the victims in Libya. This would be true even if Congress authorizes the war as required under Article I of the United States Constitution. The authorization to go to war must come from someone else other than an American political official or institution.

Hence, President Obama sought international authorization for the war in Libya. True, he sought that authority for pragmatic reasons. A coalition meant shared burdens and (Obama believes) a quick way out of Libya. But the authorizations by the U.N. Security Council and earlier by the Arab League also could be seen as giving legitimacy to the enterprise. Those authorizations meant the United States could go to Libya as a true protector of the oppressed.

If you doubt any of this, examine closely what the President has said about the war. In his speech, the rebels become victims at the mercy of an oppressor. Congress gets a fleeting mention related to consultation about, rather than authorization of, war. True legitimacy for the war comes from a “U.N. mandate and international support.” In his letter to Congress announcing the war, the first sentence reads “at my direction, U.S. military forces commenced operations to assist an international effort authorized by the United Nations (U.N.) Security Council and undertaken with the support of European allies and Arab partners, to prevent a humanitarian catastrophe…” Here again the legitimacy for the war comes the United Nations, the European allies, and the Arab League. Congress has neither power to deny the president nor legitimacy to bestow on his work.

There is much to say about these reasons for war. Some people might see in Libya a civil war between two armed gangs. Lacking the frame of oppressor and victims, they will be less willing than the President to assume that the people in the territory called Libya wear either black or white hats. We may learn to our cost that our new allies are victims now and oppressors later. If we take the President seriously, we will be obligated to make war against them, too.

We have now taken on a default obligation to help every victim and to punish every oppressor throughout the world. We have two constraints on fulfilling that obligation. The first, mentioned by the president, is costs. Eventually the financial markets may limit our efforts on behalf of victims. Second, and more important legally, a president must seek authorization for war from the United Nations, the European union, the Arab League or….well, anyone except the United States Congress.

It is not just that this president, like others before him, ignored Article I of the Constitution. Nor is this president the first to shun moral complexity in favor of a Manichean outlook. President Obama is the first, however, to assert that his broad powers to initiate war should be limited primarily by people who are outside the American social compact. On this account, sotto voce, the Constitution is not just ignored. It is irrelevant.