World War II started in much the same fashion as World War I with Germany militarily ‘annexing’ neighboring countries. With an estimated 60-85 million deaths and hundreds of millions more wounded, WWII remains the deadliest military conflict in history.
Great battles were waged in what were named the European and Asian theaters. Global fighting involved a plethora of countries participating in massive destruction with anti-German and anti-Japanese forces called the Allies and the countries supporting and including Germany and Japan, called Axis powers.
A partial accounting of Allies included the United States, Soviet Union (now Russia), England, Canada, Australia, France, Burma, China, Columbia, Indonesia, India, Pakistan, Greece, Iran, Iraq, Norway, Peru, Panama, Philippines, Yugoslavia, Saudi Arabia, Syria, Turkey, the Netherlands, most African countries and many more.
The Axis included Germany, Italy, Japan, Finland, Hungary, Romania and Thailand among others.
Germany invaded Poland, France, Belgium, Norway, Yugoslavia, England and Denmark with Japan invading China then joining Germany in declaring war on the United States.