A preventive war is a war initiated to prevent another party from acquiring a capability for attacking. The power being attacked has either a latent threat capability or has shown through its posturing that it intends to follow suit with a future attack. Preventive war aims to forestall a shift in the balance of power by strategically attacking before the balance of power has a chance to shift in the direction of the adversary. Preventive war is distinct from preemptive war, which is first strike when an attack is imminent. Preventive war undertaken without the approval of the United Nations is illegal under the modern framework of international law, though Robert Delahunty and John Yoo from the George W. Bush administration maintained in their discussion of the Bush Doctrine that these standards are unrealistic.
Advocates of preventive war have ranged from Posadist Communists, who argued for war to destroy capitalism, to western neo-conservatives such as George W. Bush and Donald Rumsfeld, who argued that preventive war is necessary in today's post September 11th world. Proponents claim it has been used throughout American history and is especially relevant in the present as it relates to unconventional war tactics and weapons of mass destruction. The National Security Strategy advocates a policy of proactive counterproliferation efforts, and preventive measures.
There is a consensus that preventive war "goes beyond what is acceptable in international law" and lacks legal basis. While the UN High Level Panel on Threats, Challenges and Change stopped short of rejecting the concept outright, it suggested that there is no right to preventive war. If there are good grounds for initiating preventive war, the matter should be put to the Security Council, which can authorize such action.
The Axis in World War II routinely invaded neutral countries on grounds of prevention, and began their invasion of Poland in 1939 by claiming the Poles had attacked a border outpost first. In 1940, Germany invaded Denmark and Norway, arguing that Britain might have used them as launching points for an attack, or prevented supply of strategic materials to Germany. Then in the summer of 1941, Germany invaded the Soviet Union, inaugurating this bloody and brutal land war by claiming a Judeo-Bolshevik conspiracy threatened the Reich. In late 1941, the British and Soviets invaded Iran to secure a supply corridor of petrol to the USSR. The Shah of Iran appealed to U.S. President Franklin D. Roosevelt for help, but was rebuffed on the grounds that "movements of conquest by Germany will continue and will extend beyond Europe to Asia, Africa, and even to the Americas, unless they are stopped by military force."
Perhaps the most famous example of preemptive war is the war in the Pacific, launched when Japan attacked military targets in Pearl Harbor. Many in the U.S. and Japan believed war was inevitable, this belief coupled to the crippling U.S. oil embargo that was rapidly degrading Japanese military capability led the Japanese leadership to believe it was better to have the war as soon as possible.
The Japanese attack on Pearl Harbor in 1941 was partly motivated by a desire to cripple U.S. naval power in the Pacific in order to allow the Empire of Japan to advance with reduced opposition from the US when it, in order to secure Japanese oil supplies, fought the British Empire and the Dutch Empire for control over the rich East Indian (Dutch East Indies, Malay Peninsula) oil-fields. In 1940, American policies and tension toward Japanese military actions and Japanese expansionism in the Far East increased. For example, in May 1940, the base of the U.S. Pacific Fleet that was stationed on the west coast of the United States was forwarded to an "advanced" position at Pearl Harbor, Hawaii.
The move was opposed by some Navy officials, including their commander, Admiral James Otto Richardson, who was relieved by President Roosevelt. Even so, the Far East Fleet was not significantly reinforced. Another ineffective plan to reinforce the Pacific was a rather late relocation of fighter planes to bases located on the Pacific islands (like Wake Island, Guam, and the Philippines). For a long time, Japanese leaders, especially leaders of the Imperial Japanese Navy, had known that the large military strength and production capacity of the United States posed a long-term threat to Japan's imperialist desires, especially if hostilities broke out in the Pacific. War games on both sides had long reflected these expectations.
A dispute over territorial waters led Egypt to expel UN forces from a pre-established buffer zone, close the Straits of Tiran to Israel, and mobilize over 1,000 tanks and 100,000 soldiers on Israel's borders. Israel could not maintain a comparable level of mobilization due to its smaller population, and so decided to strike first, launching the Six-Day War which has been described as both preemptive (Michael Waltzer) and preventive.
The 2003 invasion of Iraq was claimed as a preemptive war by the Bush administration. Supporters of the war have argued that it was justified, as Iraq harbored Islamic terrorist groups that share a common hatred of Western countries and was suspected to be developing weapons of mass destruction (WMD).
In support of an attack on Iraq, U.S. President George W. Bush stated in an address to the United Nations on September 12, 2002, that the Iraqi "... regime is a grave and gathering danger." However, despite several years of occupation, the weapons of mass destruction he alleged have not been found.