World War 2 was a global war, the largest and deadliest of all wars in human history, with over 60 million people killed, including millions of civilians who were systematically mass-murdered. Many more millions were injured, imprisoned, starved, deported, became refugees, subjected to forced labor, and so on. Entire countries were physically devastated by the war. On the "positive" side, many people, from individuals to entire nations, rose to the challenge and demonstrated unprecedented courage, determination, endurance, diligence, cooperation, and a desire to prevail over evil and hardship.
World War 2 was started by Adolf Hitler, then the dictator of Germany. Hitler, who was an extreme racist, a Nazi, declared his aggressive expansionist intentions years in advance, in a book and in speeches, but the German people elected him to be the chancellor of Germany. As chancellor, he immediately abolished democracy, as he promised in advance, becoming a totalitarian dictator, and turning Germany to a police state of unlimited brutality, which physically eliminated entire "undesired" population groups. Under his leadership, Germany's resources were dedicated to rapid military buildup for the intended war.
Hitler's Germany's allies, called The Axis powers, were Japan and Italy. Both nations then had militarist, aggressive, expansionist regimes, though not ideologically racist as Hitler's regime. Several east European nations also allied with Germany, either following its ideological example, or to protect themselves from their other aggressive expansionist neighbor, the Soviet Union, which Hitler marked from the beginning as his main target.
The countries which allied against the aggression of the Axis powers, were simply called The Allies. These initially included Great Britain and France, and the British Commonwealth. Other European nations joined as they were attacked by the Axis, but all were invaded and occupied by Germany, except Great Britain, which for an entire year of the war stood bravely alone in Europe against the Axis. Then, after also being attacked by the Axis powers, the United States of America and the Soviet Union joined The Allies.
World War 2 started with a German invasion of Poland, continued with German invasions of most of Europe, expanded to north Africa and the middle east, and to fierce naval warfare in the Atlantic Ocean and the Mediterranean. When Japan, which already invaded China, also attacked The Allies, the war expanded to east Asia and the Pacific Ocean, literally becoming a global war.
All three Axis powers, Germany, Japan, and Italy initially exploited British and French appeasement, American isolationism, and other factors, to invade and conquer neighboring countries without triggering a large war. In Germany's case, Hitler cunningly used the threat of war to achieve these conquests without a fight.
In order to stop Hitler from invading Poland, which Hitler openly marked as his next victim, Britain and France officially pledged to join the war at once if Poland would be attacked. But Hitler was not deterred, and on September 1, 1939, Germany started World War 2 by invading Poland. In 1941, Germany invaded the Soviet Union and Japan attacked the United States of America, forcing them to join the war. In 1945, after six years of war, the largest and deadliest of all wars, the Axis powers were finally defeated by the Allies. On May 8, 1945, Germany surrendered, ending the war in Europe, and on August 15, 1945, Japan surrendered, and World War 2 ended.
World War 2 was fought because the militarist aggressive expansionist leaders of the Axis countries, especially Adolf Hitler, noticed the mental fatigue, the unwillingness to resist, the internal divisions and weaknesses, and the neglect of the armies, in neighboring countries, and fully exploited that. These conditions, which Hitler tested several times in increasing scale, eliminated the deterrence that normally keeps such aggressors from starting wars.
The combination of an extreme aggressor like Hitler, the world's most effective army at his command and the lack of deterrence, is what caused the war. Europe's unwillingness to resist Hitler's early aggression, or even to prepare for war, is what caused World War 2, because it signaled to Hitler, and to the other Axis leaders, that they will not be opposed.
The initial military power of each of the Allies was much smaller than its full potential, while the aggression of Germany and Japan was backed by formidable military efficiency, the result of centuries of militaristic traditions, combined with the latest weapons and tactics. But the Allies had 3 factors which let them gradually gain the upper hand, determination, the peoples of the Allies were determined to fight until victory. Distance, they had significant sea or land barriers which protected them from being quickly overrun, and therefore provided the time they needed to fully mobilize. And of course resources, the Allies had enormous military potential, human, industrial, and technological, and they pushed hard and fast to mobilize it.
The Axis leaders correctly interpreted the Allies pre-war weaknesses, but greatly under-estimated their military potential and determination, once forced to fight. In 1941, the Soviet Union lost millions of soldiers and civilians, and still stopped the German invasion, mobilized millions more, and by 1943 clearly gained the upper hand. With the industry, already in 1940, British aircraft production alone exceeded that of Germany. Russia produced several times more tanks than Germany, and when the US joined the war, within a year it produced more weapons than the Allies could use, or the Axis could destroy. Germany developed a wide array of new military technologies, and Japan built the world's most powerful warships and submarines. But Russia developed the world's best tank, Great Britain decisively defeated Germany in all aspects of electronic and information warfare, and the US focused on war-winning technologies like mass-production, long-range bombers, and the atomic bomb.
World War 2 was fought with the weapons of the industrial age: tanks, guns, aircraft, warships and submarines, and millions of soldiers. But World War 2 also saw the debut of most of the military technologies that we have today. The first computers were built, for code-breaking. Missiles, guided weapons, and other means for precision attack, appeared in World War 2. Jet engines, and long-endurance submarines, which revolutionized air and naval warfare, appeared in World War 2.
Hitler saw the destruction of his vision of German world domination, and committed suicide. Both winners and losers suffered staggering losses, over 60 million killed in total, and the physical and financial damages of the war. Most countries needed time to recover, and that resulted in the post-war dissolution of the European colonial empires. The Soviet Union, also a country with a global domination, communism, dominated all eastern Europe, for decades. But the fact that the Soviet Union suffered the heaviest losses in World War 2, over 20 million soviet soldiers and civilians, played a key role in soviet leader’s decision to avoid starting a 3rd world war for their ideology. And in a post-war world armed with enough nuclear weapons to annihilate mankind, the living memory of those who died in the war, saved mankind from destruction.