The global conflict, now known as the Second World War, was the deadliest confrontation witnessed in human history, involving numerous nations across the globe. The participants in the war were primarily in two alliances—the Allies and the Axis powers. The war involved several theaters spread across different continents and even included conflicts already underway based on later categorization.

The precipitation of the war had numerous factors, such as the aftermath of the First World War and the impact it had on the geopolitical psyche of the defeated Central Powers. The long-lasting effects of the treaties and harsh actions taken by the winners of the first global conflict contributed significantly to future aggression, especially from Nazi Germany. Additionally, the Great Depression, revanchist ideologies, and fascism's fostering of imperialistic ambitions were also major causes of this conflict.

The war officially began with the German invasion of Poland on September 1, 1939; however, one could also argue that the hostilities commenced with the start of the Second Sino-Japanese War in July 1937. Despite differences of opinion, it is evident that the Second Sino-Japanese War undeniably became a part of the larger theater of World War II when hostilities in Europe commenced. Moreover, the signing of the Tripartite Pact in September 1940 effectively placed Japan among the Axis powers.

In Europe, a combination of various political factors fomented considerable unrest. This included the revolutions in Russia and Germany and the subsequent rise of National Socialism in the latter. Moreover, the Italian Fascist Movement was another disruptor of peace in Europe that led to growing imperialism in the country and beyond. As for Asia, a rising Japan and its monarchy that looked outward for resources created an expansionist power that would later join the broader conflict. The economic factors for war primarily included the Great Depression, which contributed significantly to various pressures on societies as well as their governments.

The conflict was characterized by several war crimes and genocides, including the Holocaust, resulting in the greatest number of human casualties ever reported during a time of war. Despite rapid Axis advances, the Allies successfully countered their adversaries’ gains following numerous grueling battles fought across Europe, Africa, Asia, and Oceania. Following Italy’s surrender and the Allied invasion in 1943, German holdings in Europe faced a bidirectional invasion and counter-offensive not long after, in 1944. With Germany offering unconditional surrender on May 8, 1945, the Allied victory in Europe was complete. However, it wouldn’t be until August 15, 1945, for Japan’s surrender and the formal signing of the treaty on September 2, 1945, which brought an end to the war in the Pacific. The conflict also witnessed the only use of nuclear weapons in human history, over Hiroshima and Nagasaki.

The world today has been shaped by the impact of World War II, and the extensive destruction it left behind prompted a massive rebuilding effort and a move toward preventing future conflicts to avoid losses in infrastructure, economic growth, and humanitarian aid. Geopolitically, the end of the war also witnessed the formal end of colonialism in several parts of the world, with imperialist powers finally giving up their overseas territories and submitting to demands for independence from their colonies.

With the US and USSR on the rise as resulting global powers, the competition between both nations would eventually lead to the Cold War, which lasted right until the dissolution of the latter in 1991. The United Nations, too, was formed in the aftermath of the Second World War and would become an organization with more capabilities and power compared to the League of Nations, which failed to prevent global conflict. Irrespective, the technological, scientific, and industrial precedents of the war permanently altered the world.