linguistic:paraphrasing, structural:reformatting, information:classification
Tripartite Pact, World War II and contemporary history (post-1945)

Facing resource scarcity due to a growing population, Japan seized Manchuria in September 1931 and put ex-Qing emperor Puyi in charge as head of the puppet state of Manchukuo in 1932. During the Sino-Japanese War (1937–1945), the loss of Manchuria, and its vast potential for military-industrial development, was a blow to the Chinese economy. After 1940, conflicts between the Kuomintang and Communists became more frequent in the areas not under Japanese control. The Communists expanded their influence wherever opportunities presented themselves through mass organizations, administrative reforms, and the land- and tax-reform measures favoring the peasants—while the Kuomintang attempted to neutralize the spread of Communist influence. The Second Sino-Japanese War had seen tensions rise between Imperial Japan and the United States; events such as the Panay incident and the Nanjing massacre turned American public opinion against Japan. With the occupation of French Indochina in the years of 1940–41, and with the continuing war in China, the United States placed a metal and oil embargo on Japan which were vital to its war effort. The Japanese were faced with the option of either withdrawing from China or seizing and securing new sources of raw materials in the resource-rich, European-controlled colonies of Southeast Asia—specifically British Malaya and the Dutch East Indies (modern-day Indonesia).

Although Japan had invaded China in 1937, the conventional view is that the World War II began on September 1, 1939, when Nazi Germany invaded Poland. Within two days the United Kingdom and France declared war on Germany, even though the fighting was confined to Poland. Pursuant to a then-secret provision of its non-aggression Molotov–Ribbentrop Pact, the Soviet Union joined Germany on September 17, 1939, to conquer Poland and divide Eastern Europe. The Allies were initially made up of Poland, the United Kingdom, France, Australia, Canada, New Zealand, South Africa, as well as British Commonwealth countries which were controlled directly by the UK, such as the Indian Empire. All of these countries declared war on Germany in September 1939.

Following the lull in fighting, known as the " Phoney War ", Germany invaded western Europe in May 1940. Six weeks later, France, in the meantime, attacked by Italy as well, surrendered to Germany, which then tried unsuccessfully to conquer Britain. On September 27, Germany, Italy, and Japan signed a mutual defense agreement, the Tripartite Pact, and were known as the Axis Powers. Nine months later, on June 22, 1941, Germany launched a massive invasion of the Soviet Union, which prompt it to join the Allies. Germany was now engaged in fighting a war on two fronts. This proved to be a mistake – Germany had not successfully carried out the invasion of Britain and the war turned against the Axis.

On December 7, 1941, Japan attacked the United States at Pearl Harbor, bringing it too into the war on the side of the Allies. China also joined the Allies, as did most of the rest of the world. China was in turmoil at the time, and attacked Japanese armies through guerilla-type warfare. By the beginning of 1942, the alignment of the major combatants were as follows: the British Commonwealth, the Soviet Union and the United States were fighting Germany and Italy; China, the British Commonwealth, and the United States were fighting Japan. The United Kingdom, the United States, the Soviet Union and China were referred as a "trusteeship of the powerful" during the World War II and were recognized as the Allied "Big Four" in Declaration by United Nations These four countries were considered as the " Four Policemen " or "Four Sheriffs" of the Allies power and primary victors of World War II. Battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, throughout China, across the Pacific Ocean and in the air over Japan. Italy surrendered in September 1943 and was split into a northern Germany-occupied puppet state and an Allies-friendly state in the South; Germany surrendered in May 1945. Following the atomic bombings of Hiroshima and Nagasaki, Japan surrendered, marking the end of the war on September 2, 1945.

It is possible that around 62 million people died in the war; estimates vary greatly. About 60% of all casualties were civilians, who died as a result of disease, starvation, genocide (in particular, the Holocaust), and aerial bombings. The Soviet Union and China suffered the most casualties. Estimates place deaths in the Soviet Union at around 23 million, while China suffered about 10 million. No country lost a greater portion of its population than Poland: approximately 5.6 million, or 16%, of its pre-war population of 34.8 million died. The Holocaust (which roughly means "burnt whole") was the deliberate and systematic murder of millions of Jews and other "unwanted" during World War II by the Nazi regime in Germany. Several differing views exist regarding whether it was intended to occur from the war's beginning, or if the plans for it came about later. Regardless, persecution of Jews extended well before the war even started, such as in the Kristallnacht (Night of Broken Glass). The Nazis used propaganda to great effect to stir up anti-Semitic feelings within ordinary Germans.

Over the course of the 20th century, the world's per-capita gross domestic product grew by a factor of five, much more than all earlier centuries combined (including the 19th with its Industrial Revolution). Many economists made the case that this understated the magnitude of growth, as many of the goods and services consumed at the end of the 20th century, such as improved medicine (causing world life expectancy to increase by more than two decades) and communications technologies, were not available at any price at its inception. However, the gulf between the world's rich and poor grew wider, and the majority of the global population remained in the poor side of the divide.