At the Nuremberg War Crimes Tribunal it was called "aggressive war." The term now more commonly used is "preventive war."
But whatever label you put on it, it's a war of aggression and a criminal act.
Steven Ratner writes in the article on "Aggression" in Crimes of War 2.0: What the Public Should Know (2007):
Aggression in international Law is defined as the use of force by one State against another, not justified by self-defense or other Legally recognized exceptions. The illegality of aggression is perhaps the most fundamental norm of modern international Law and its prevention the chief purpose of the United Nations. Even before the UN, the League of Nations made the prevention of aggression a core aim; and the post-World War II Allied tribunals regarded aggression as a crime under the rubric crimes against peace.
The most authoritative definition comes from the UN General Assembly [in 1974]. (The UN Charter never defines the term, instead banning the threat or use of force.) ...
The definition begins by stating that "[t]he first use of armed force by a State in contravention of the Charter" constitutes prima facie evidence of aggression. The definition is somewhat Limiting, and perhaps circular, in that the first use of force by a State would not be aggression if undertaken in a way consistent with the Charter. Thus, for example, the deployment of U.S. forces to Somalia in 1992, while the first use of force, would not be aggression because it was authorized by the Security Council under Chapter VII of the Charter. A number of States have accepted that a State's first use of force to extricate its citizens from another State when they are in imminent danger, and the other State is not able to protect them, is not aggression (e.g., Israel's 1976 Entebbe raid) and may be a form of self-defense. Some scholars and human rights activists have advocated a broader right of non-UN-approved intervention to prevent large-scale human rights abuses.
Moreover, most States seem willing to accept, albeit tacitly, a limited right of a State to a first use of armed force - so-called anticipatory self-defense - where it is facing a certain, imminent, and catastrophic attack, such as was faced by Israel on the eve of the June 1967 war. However, the assertion by the United States government in its 2002 National Security Strategy of a broader right of preemptive self-defense to deal with "rogue States and terrorists" possessing weapons of mass destruction elicited significant criticism internationally. Foreign governments and others also condemned the war in Iraq in 2003 in part because they rejected U.S claims that the action was authorized by prior Security Council resolutions (the U.S. did not make a legal argument that the war was taken as a matter of self-defense) and concluded that it was thus aggression under the Charter.
The General Assembly's definition also offers an illustrative list of acts of aggression: invasion, attack, or occupation of whatever duration; bombardment; blockade; attack on another State's armed forces; unauthorized use of military forces stationed in a foreign State; allowing territory to be used for aggression; and sending armed bands or similar groups to carry out aggression or substantial involvement therein.
Acts of aggression such as these trigger the two key lawful uses of force mentioned in the Charter: (a) individual or collective self-defense; and (b) force approved by the UN itself. Thus, the Iraqi invasion of Kuwait triggered the right of Kuwait and its allies to engage in self-defense, as well as the right of the UN to approve the use of force against Iraq under Chapter VII. [bolding in original; my emphasis in italics]
It's very nice that President Obama is asking Congress to approve this criminal act of aggression before he undertakes it. As Chas Freeman writes in Don't Just Sit There, Bomb Something LobeLog 09/01/2013, "There is a chance that, in the course of debating the order to attack Syria, someone will actually read our Constitution (Article I, Section 8, Clause 11) on how wars are to be legally authorized."
And maybe they will. But international law is also binding on the US. And Congress giving some kind of a formal green light to a criminal war of aggression doesn't make it any less a war of aggression.
I'm not kidding myself about how much the criminality of the act will affect its public popularity. With very rare exceptions, the Republican Party from its Members of Congress to its voting base sneer at the concept of international law. Most of them even admire the willingness of a President to break the law in the course of starting a war, as we saw in the invasion of Iraq in 2003. Oliver North became a hero to many grassroots Republican because he broke the law in selling missiles to the Islamic regime in Iran and providing funding for murderous terrorists in Nicaragua.
This is not only true of the Rand Paul/Tea Party/Libertarian/John Birch Society Republicans. It's more true of them. They despise the UN and international law and are militantly nationalistic. Rand Paul has criticized the idea of attacking Syria because bombing might kill some Christians and not just them thar Muslims. But it's not because the Tea Partiers recognize any validity to international law or have any respect for it. The closest they might come would be to cite international law as war propaganda to justify an American war. But you will look long and hard for Tea Partiers to object to an American act of war on the grounds that it violates international law.
Large numbers of Democrats are also in practice indifferent to the laws of war banning wars of aggression when it comes to deciding whether to authorize or support an American military action. Especially when it's a Democratic President asking for support for an illegal act of war. Democrats in Congress have again and again been willing to authorize or ignore criminal acts and war crimes in the name of the Global War on Terrorism, whether it's surveillance, torture, targeted assassinations, drone attacks in countries with which the US is not at war, indefinite detentions of "enemy combatants" without either formal charges and trial or recognition of them as prisoners of war.
Many Democrats also are advocates of "humanitarian" intervention. Although the idea of humanitarian war is about as ridiculous an oxymoron as one could imagine. The concept is a highly problematic one. David Rieff and Anthony Dworkin write about it in the "Humanitarian Intervention" item of Crimes of War 2.0, noting, "Humanitarian intervention is at once an immensely powerful and a terribly imprecise idea."
And there is a world of difference between humanitarian aid for refugees or other war victims and military intervention on alleged humanitarian grounds. In the case of an "humanitarian war" in Syria by the US and France, only the United Nations could provide a clear legal basis for it. As Rieff and Dworkin writes, "It seems generally accepted that the Security Council can declare anything it likes to be a 'threat to international peace and security,' subject not to any genuinely objective constraints of law [because there is no clear definition in international law of what qualifies] but only to the political vetoes of its permanent members." And they note, "In practice, humanitarian intervention has often served as a justification for States to act in conflicts where there is no domestic support for more straightforward political interventions."
For instance, you can count on Republicans to start showing great humanitarian concern for the rights of women in Muslim countries they want to bomb or invade. After the bombing starts, that concern rapidly fades. Though it's become standard war publicity to paint some girls' schools or something to show what nice things Americans are doing in countries where we're killing people in war.
The use of poison gas is a violation of international law, too. But that fact alone does not give the US the right in international law to start a war in the name of enforcing it.
I haven't seen any polls that would tell me whether a majority of Americans clearly care if our wars are legal in international law or not. But it matters. And whether or not the Syrian government is a nice one or a murderous one is not the critical factor. Count me as not a cheerleader for a war of aggression on Syria. Even a "limited" one.
UN Secretary-General Ban Ki-moon addressed the legal issues on Tuesday (PRESS ENCOUNTER BY SECRETARY-GENERAL BAN KI-MOON AT UNITED NATIONS HEADQUARTERS UN Department of Public Information 09/03/2013):
As I have stressed repeatedly, if confirmed, any use of chemical weapons by anyone under any circumstances would be a serious violation of international law and an outrageous war crime.Tags: preventive war, syria, syrian civil war, war crimes, war of aggression
Almost a century ago, following the horrors of the First World War, the international community acted to ban the use of these weapons of mass destruction. Our common humanity compels us to ensure that chemical weapons do not become a tool of war or terror in the twenty-first century. Any perpetrators must be brought to justice. There should be no impunity.
Bearing in mind the primary responsibility of the Security Council, I call for its members to unite and to develop an appropriate response, should the allegations of use prove to be true. The Security Council has a duty to move beyond the current stalemate and show leadership. This is a larger issue than the conflict in Syria; this is about our collective responsibility to humankind. ...
Question: On behalf of the UN Correspondents Association, thank you, Secretary-General, for the briefing, and we wish you well on the G20 meeting. My question is, since you are talking about an end to impunity and you are also talking about the primacy of the Charter, which would prohibit any military strike without UN Security Council authorization and with a stalemate in the Security Council, what is it that you are proposing? What’s in the toolbox of the UN to avoid that kind of confrontation to end impunity, and do you think the inspectors’ report will be out before the U.S. Congress convenes? Thank you.
Secretary-General: As I have repeatedly said, the Security Council has primary responsibility for international peace and security. For any course of actions in the future, depending upon the outcome of the analysis, the scientific analysis, will have to be considered by the Security Council for any action. That's my appeal — that everything should be handled within the framework of the United Nations Charter. The use of force is lawful only when in exercise of self-defence in accordance with Article 51 of the United Nations Charter and/or when the Security Council approves such action. That is the firm principle of the United Nations. And as I said again for your second question, our mandate to investigate the other allegations of chemical use remains unchanged and when we are ready, we will send, dispatch, our mission again to Syria for the final report. The timing will have to be considered later on, depending upon the situation. [my emphasis in italics]