On 7 December 1941 at 7:55am, two waves of hundreds of Japanese aircraft launched their deadly attack on the US Pacific Fleet moored at Pearl Harbour on Oahu Island, Hawaii.
The raid only lasted about two hours, but its effects were devastating. Over 2,400 Americans were killed, with another 1,178 injured (under 100 Japanese were killed), 5 battleships were sunk, 16 more damaged and 188 aircraft had been destroyed.
This Japanese offensive marked the start of the war in the Pacific – the following day, President Roosevelt signed the official declaration of war against Japan. On 11 December when Germany and Italy declared war on the US, Congress reciprocated, sealing America’s entry into World War Two – and ultimately dramatically altering its course.
What were the reasons for Japan’s surprise attack on the US Fleet at Pearl Harbour? And was the attack really such a surprise?
Tensions between Japan and America had been mounting for decades
As an island nation, isolated from the rest of the world for much of its history, Japan decided to embark on a period of aggressive expansion in the early 20th century. This followed its two successful wars (against China 1894-95, and the Russo-Japanese War in 1904-05), as well as Japan’s successful role in supporting the Allies in the First World War by securing the sea lanes in the West Pacific and Indian Oceans against the Imperial German Navy.
The Great Depression of the 1930’s not only hit America – its economic effects were felt around the world. Indeed the mass unemployment it created played a role in Hitler’s rise to power. Japan’s aim to expand in Asia and the Pacific meant they had an increased need for natural resources like oil, minerals and steel, yet they too were affected by the Depression, and tried to ease their demographic and economic woes by taking over the Chinese import market.
On 19 September 1931, Japan staged an incident at a railway station in Manchuria, which it used as an excuse to invade the mineral-rich Chinese province (remaining there until 1945). This aggression was strongly condemned by the League of Nations, prompting Japan to withdraw its membership and continue its expansion throughout the Chinese mainland. This led to the Second Sino-Japanese war in July 1937, following a clash at the Marco Polo Bridge in Beijing.
America also had an interest in the natural resources Japan sought, and as Japanese aggression increased, its relations with the US deteriorated.
Historically, Japan had relied on America to supply many resources, but alarmed by Japanese aggression in China, America allowed a commercial treaty dating from 1911 to lapse in January 1940. America also began placing restrictions on doing business with Japan and freezing Japanese assets in the US.
America was trying to stop Japan’s global expansion
Increasingly alienated, Japan joined the Tripartite Pact, forming an alliance with Nazi Germany and Fascist Italy in September 1940 who were already at war with the Allies. Although officially neutral, American sympathies clearly lay with the Allies. The Tripartite Pact would mean supplies to Japan would indirectly be helping Italy and Germany, so further US embargoes followed – further worsening Japan and America’s already strained relations.
On top of its economic considerations, Japan’s early military success and inherent sense of racial superiority led them to believe they deserved to dominate Asian politics.
After Japan invaded French Indochina in September 1940, they didn’t occupy the southern area immediately after, worried that such a move would be inflammatory to its relations with the UK and America. However, after the Nazi invasion of the Soviet Union in June 1941, Japanese high command concluded that as the Soviets were now tied down, a “strike south” would solve Japan’s problems.
To prepare for an invasion of the Dutch East Indies, Japanese troops invaded southern French Indochina on 28 July 1941. America reacted by imposing further economic sanctions on Japan, including trade embargoes on aircraft exports, oil and scrap metal, among other key goods.
Japan was particularly heavily reliant on oil imports (importing around 80% of the oil it needed) – without this key import, Japan’s military couldn’t function effectively, and thus these trade embargoes were a further enormous source of tension, deteriorating US/Japanese relations significantly.
Oil negotiations between America and Japan continued without any resolution, and by late 1941, the US had ended practically all commercial and financial relations with Japan. The US had hoped the embargoes would curtail Japan’s desire to expand its influence, however they had the opposite effect, serving to convince Japan to stand its ground. Japan viewed America’s actions as interferring in Asian affairs.
Destroying the American base at Pearl Harbor would mean Japan could control the Pacific
As tensions with America increased, Japan thought war with the US had become inevitable. It knew a full-scale invasion of South-east Asia would prompt war with America, but needed time to conquer crucial targets like the Philippines, Burma and Malaya.
America had made Pearl Harbor the main base for its Pacific Fleet in May 1940. As Hawaii was over 4,000 miles away from the Japanese mainland, they didn’t expect the Japanese to attack Pearl Harbor first, and consequently the base was left relatively undefended.
Japanese Admiral Yamamoto Isoroku knew that Japan could not conquer, or even defeat, the United States. Instead it aimed to destroy the Pacific Fleet through quick, coordinated attacks from their exisiting Pacific bases, overwhelming Allied forces.
Japan hoped this would remove America from the Pacific equation long-enough for Japan to invade South-east Asia and create and maintain a stronghold stretching across the Pacific Rim. This would allow Japan to secure the resources it needed so desperately and crush the US Navy’s morale, meaning America would hopefully accept defeat and seek a negotiated peace.
Japan needed to destroy America’s navy as quickly as possible
Despite the area being made up of islands, Allied air power in the Pacific was weak. Knowing the odds were stacked against them, the element of surprise by attacking Pearl Harbour seemed to Japan like their only chance of victory.
Despite information and warnings from Allied codebreaking operations and diplomatic sources, the US military was completely unprepared for the surprise attack, expecting any Japanese attack to be on American targets in Thailand or the Dutch East Indies, rather than this close to home.
Although a stunning short-term tactical success for Japan, the attack had ultimately failed to completely destroy the US Pacific Fleet. Usually stationed at Pearl Harbor, by chance, 3 US aircraft carrier fleets had been at sea that day, and survived unscathed – a critical missed opportunity by Japan.
Whilst operationally brilliant, the attack on Pearl Harbour was strategically disastrous. Rather than crushing morale, it had the effect of uniting the American population behind the war effort. The onset of the Pacific War now also pitted Japan in a total war against the largest economy in the world.