One of the causes of World War II was a worldwide economic depression. The stock market crash that left America reeling through the 1930s was mirrored in economies all over the globe. People became desperate for jobs, food, resources, and hope. This type of desperation makes room for a strong political leader to come into the void of hope and lead people astray. People become desperate for someone, anyone, to restore economic stability. When Hitler promised Germany that he could restore the country to its former strength, people wanted to believe him. Therefore, they were willing to follow his leadership, which proved devastating.
Germany and Hitler were causes of a worldwide war, as well. Germany was bent on gaining back what they had lost through the Treaty of Versailles. Hitler invaded parts of France, Austria, and Czechoslovakia. Aligning with Italy and Japan further strengthened the country and placed this group in direct opposition with other countries around the world.
Germany wasn't the only country looking to expand its strength. Japan, too, found itself in need of further resources and began invading China and Vietnam. The threat Japan inflicted on areas in Asia, particularly the Philippines and Guam which were United States territories, became an increasing source of tension around the world. Japan was increasingly seen as hostile and unpredictable.
Other factors, such as the failure of the League of Nations, contributed to the eventual war, but these are the primary factors that influenced the onset of war, in my opinion (which is always debatable by historians). From this, we can learn that people in desperate situations resort to desperate measures—both leaders of countries and the citizens of those countries. People follow tyrannical rulers because they feel they have no other hope of survival. It's therefore important to share with others the causes of great disasters, such as World War II, so that people understand the importance of working through conflict peacefully and the importance of sharing the earth's resources so that people don't feel this sense of dire desperation which can ultimately lead to world-changing, global conflict.