... like I'm 5 years old
World War II began because of unresolved issues from the first World War, and the desire for power and control by certain nations. Imagine a group of children playing together. One child, let's call him Germany, is being blamed and punished for a fight that happened previously (World War I). He's not happy about this and starts to misbehave, trying to get control of the toys and the playground. Two other children, Italy and Japan, decide to join Germany because they also want more control. They start bullying other children, like Poland, and taking their toys. Finally, the rest of the children (Allied Forces) decide they've had enough and step in to stop the bullies. This is a very simple way to look at how World War II started.
In this analogy, the playground is the world, the children are the countries, the toys represent resources and control, and the bullies starting fights is similar to countries invading others.
... like I'm in College
World War II began as a result of the Treaty of Versailles that ended World War I. The treaty placed full blame for the war on Germany, causing economic and political instability. As Germany dealt with economic depression and hyperinflation, resentment grew. This led to the rise of Adolf Hitler and the Nazi party, who tapped into this national resentment.
At the same time, Italy, under the fascist rule of Benito Mussolini, sought to establish a new Roman Empire. Japan, under Emperor Hirohito, pursued territorial expansion in Asia. These three nations, Germany, Italy, and Japan, formed the Axis Powers.
The war officially started when Germany invaded Poland in September 1939. Britain and France, who had agreements with Poland, declared war on Germany. This led to a global conflict involving most of the world's nations.
Imagine a giant Lego set, representing the world in the late 1930s. Each country is a different color brick. The German, Italian, and Japanese bricks aren't happy with the way they've been arranged after World War I, so they start moving around, pushing other bricks out of their way.
Germany (a big red brick) pushes into the Austria and Czechoslovakia bricks. Italy (a green brick) takes over the Ethiopia brick. Japan (a white brick) moves into the Manchuria and China bricks.
Then, the big red brick (Germany) pushes into the Poland brick. This move makes the Britain and France bricks (blue and red-white-and-blue) declare it's time to reset the Lego set. They start pushing back against Germany, Italy, and Japan. Other bricks, representing countries from all over the world, join in to help restore order to the Lego set. This giant Lego war represents World War II.
... like I'm an expert
World War II's origins can be traced to the aftermath of World War I and the Treaty of Versailles, which the German Weimar Republic felt was unjust, leading to widespread discontent. The global economic depression of the 1930s exacerbated this, leading to political instability and the rise of extremist ideologies.
In Germany, this culminated with the Nazi Party led by Adolf Hitler gaining power in 1933. Hitler rejected the Treaty of Versailles and pursued an aggressive foreign policy, leading to the Anschluss with Austria and the Sudetenland Crisis with Czechoslovakia as precedents to war.
Simultaneously, Italy under Mussolini sought to create an Italian Empire, leading to the invasion of Ethiopia. Japan, pursuing its own imperialistic goals in Asia, invaded Manchuria and later China, setting the stage in the Pacific Theater.
The official start of the war can be marked by the German invasion of Poland on September 1, 1939, and the subsequent declarations of war by Britain and France. This was the beginning of a six-year global conflict.