Unquestionably, if Turkey or Jordan been attacked, we would be obligated under our treaties with these countries to defend them. But Syria's attack against its own civilians is part of a civil war. The justification for military intervention is based entirely on the premise that certain actions are so heinous that they require a response by the international community.
Secretary Kerry makes this case in his remarks here. First, it is not a unilateral decision by the United States, but one made in concert with other governments that share these views. Second, as Kerry observes:
"The meaning of this attack goes beyond the conflict in Syria itself, and that conflict has already brought so much terrible suffering. This is about the large-scale, indiscriminate use of weapons that the civilized world long ago decided must never be used at all, a conviction shared even by countries that agree on little else. . . .
And there is a reason why, no matter what you believe about Syria, all peoples and all nations who believe in the cause of our common humanity must stand up to assure that there is accountability for the use of chemical weapons so that it never happens again."
The counter argument is that military violence cannot be justified except in self-defense (including defense of an ally), and that the use of chemical weapons is no different than the use of conventional weapons -- which has already killed more than 100,000 people in Syria. But I submit that it is rational and moral to draw the line at the use of "weapons of mass destruction" against civilian populations.
A country may fight rebellion using conventional means, and suffer civilian casualties. This is an unfortunate inevitability of war. Nor is it easy to tell, at a distance, whether alleged massacres of civilians with conventional weapons took place, or under what circumstances -- whether by order or authorization of the central authority or in the heat of the moment by soldiers or commanders on the ground. This is not to say that such acts are excusable, but that they are not of a character or nature so as to warrant intervention by the governments of the world as against a recognized sovereign power. It has long been the province of sovereigns to wage war against one another or against armed rebellion, and the danger of civilian casualties has long been recognized and accepted as one of the risks of the exercise of the sovereign right to wage war and suppress rebellion.
But indiscriminate use of weapons of mass destruction is in entirely different category precisely because they are weapons of mass destruction. There is no capability to limit civilian casualties. Nor can it be demonstrated that employment of such weapons occurred in an unauthorized manner. As a general rule, countries keep close watch on their stockpiles of chemical weapons, and do not issue them casually.
Accordingly, it is rational to distinguish between the use of weapons of mass destruction and the use of conventional weapons. The question that remains is both the morality of military response to the use of such weapons against civilians and the feasibility of such response.