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. They 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.
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".
Whilst The Nazi invasion of Russia in 1941 is usually considered an aggressive war that started with a surprise attack on a former ally, Hitler's apologist David Irving in his book Hitler's War controversially argued that this was a Preventative war.
Though normally considered an act of aggression by Japan, apologists for Imperial Japan have argued that this war was Preventative as follows.
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 of 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 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.