A preventive war or preventative war is a war initiated under the belief that future conflict is inevitable, though not imminent. 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.  Due to the speculative nature of preventive war, in which the adversary may or may not be a future threat, preventive war is considered an act of aggression in international law.  Some apologists for aggressive wars have argued them to be justified as Preventive. Arguments as to whether a war was a preemptive war or a preventive war, or a preventive war rather than an war of aggression can be very controversial as they are in effect arguments as to whether those wars were justified or not.
In the modern era, advocates of Preventative war tend to be from the political fringes, ranging from Posadist Communists arguing for war to destroy Capitalism; to western neo-conservatives such as George W. Bush and Donald Rumsfeld, who argue that preventive war is necessary in today's post September 11th world. Proponents claim it has been used throughout American history and 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.[3 ]
Legal scholars generally agree that preventive war is not legally distinct from aggression, "the supreme crime" in international law. Commentators as diverse as Dwight Eisenhower and Noam Chomsky have argued that accepting one preventive war would open the floodgates to all preventive wars, reducing the world to "the law of the jungle". Critics argue this 'war-at-will' society creates an environment where war can be easily justified.
Both Axis and Allies in World War II invaded neutral countries on grounds of prevention. 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. In 1941, the British and Soviets invaded Iran to secure a supply corridor into Russia. The Shah of Iran appealed to 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".
Though normally considered an act of aggression by Japan, apologists for Imperial Japan have argued that this war was preventative.
The Japanese attack on Pearl Harbor in 1941 was motivated by a desire to remove U.S. naval power from the Pacific to allow the Empire of Japan to advance with reduced opposition into the rich Southern Resource Area (the Dutch East Indies, the Malay peninsula, the Philippines, etc). 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 in consequence 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 (e.g., 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 buffer zone, close the Straits of Tiran to Israel, and mobilize over 1,000 tanks and 100,000 soldiers on the Israel's borders. Israel could not maintain a comparable level of mobilization due to its smaller population, and so decided to strike first, and launched the preemptive Six-Day War.
The 2003 invasion of Iraq was claimed as a preemptive war by the Bush administration but has been widely criticised as an aggressive or preventive war, either by those who contend that Iraq did not pose an imminent military threat to the USA or even as an attempt to "rob Iraq's Oil"  .
The war's supporters have argued that the war was justified, as Iraq had Islamic terrorist groups that share a common hatred of Western countries. The war's supporters also state that these groups might, in the future, threaten international peace and security and, specifically, Europe and the United States.
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 alleged WMDs have not been found .