The main causes of World War II were nationalistic tensions, unresolved issues, and resentments resulting from the World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 and the interwar period

Interwar period

Interwar period can refer to any period between two wars. The Interbellum is understood to be the period between the end of the Great War or First World War and the beginning of the Second World War in Europe....

 in Europe, plus the effects of the Great Depression

Great Depression

The Great Depression was a severe worldwide economic depression in the decade preceding World War II. The timing of the Great Depression varied across nations, but in most countries it started in about 1929 and lasted until the late 1930s or early 1940s...

 in the 1930s.

The culmination of events that led to the outbreak of war are generally understood to be the 1939 invasion

Invasion of Poland (1939)

The Invasion of Poland, also known as the September Campaign or 1939 Defensive War in Poland and the Poland Campaign in Germany, was an invasion of Poland by Germany, the Soviet Union, and a small Slovak contingent that marked the start of World War II in Europe...

 of Poland

Poland

Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

 by Germany

Nazi Germany

Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...

 and Soviet Russia

Soviet invasion of Poland

Soviet invasion of Poland can refer to:* the second phase of the Polish-Soviet War of 1920 when Soviet armies marched on Warsaw, Poland* Soviet invasion of Poland of 1939 when Soviet Union allied with Nazi Germany attacked Second Polish Republic...

 and the 1937 invasion

Second Sino-Japanese War

The Second Sino-Japanese War was a military conflict fought primarily between the Republic of China and the Empire of Japan. From 1937 to 1941, China fought Japan with some economic help from Germany , the Soviet Union and the United States...

 of the Republic of China

Republic of China

The Republic of China , commonly known as Taiwan , is a unitary sovereign state located in East Asia. Originally based in mainland China, the Republic of China currently governs the island of Taiwan , which forms over 99% of its current territory, as well as Penghu, Kinmen, Matsu and other minor...

 by the Empire of Japan

Empire of Japan

The Empire of Japan is the name of the state of Japan that existed from the Meiji Restoration on 3 January 1868 to the enactment of the post-World War II Constitution of...

.

These military aggressions were the result of decisions made by the authoritarian ruling Nazi elite in Germany and by the leadership of the Kwantung Army in Japan.