Hitler sought Lebensraum (" living space ") for the German people in Eastern Europe and his aggressive foreign policy is considered to be the primary cause of the outbreak of World War II in Europe . He directed large-scale rearmament and on 1 September 1 939 invaded Poland , resulting in Britain and France declaring war on Germany . In June 1 941 , Hitler ordered an invasion of the Soviet Union . By the end of 1 941 , German forces and the European Axis powers occupied most of Europe and North Africa . In December 1 941 , he formally declared war on the United States , bringing them directly into the conflict . Failure to defeat the Soviets and the entry of the United States into the war forced Germany onto the defensive and it suffered a series of escalating defeats . In the final days of the war during the Battle of Berlin in 1 945 , he married his long-time lover Eva Braun . Less than two days later on 30 April 1 945 , the two killed themselves to avoid capture by the Soviet Red Army and their corpses were burned . Under Hitler ' s leadership and racially motivated ideology , the Nazi regime was responsible for the genocide of at least 5.5 million Jews and millions of other victims whom he and his followers deemed Untermenschen ( sub-humans ) or socially undesirable . Hitler and the Nazi regime were also responsible for the killing of an estimated 1 9.3 million civilians and prisoners of war . In addition , 29 million soldiers and civilians died as a result of military action in the European theatre . The number of civilians killed during the Second World War was unprecedented in warfare and the casualties constituted the deadliest conflict in human history .
World War II ( often abbreviated to WWII or WW2 ), also known as the Second World War , was a global war that lasted from 1 939 to 1 945 , although related conflicts began earlier . It involved the vast majority of the world ' s countries — including all of the great powers — eventually forming two opposing military alliances : the Allies and the Axis . It was the most widespread war in history , and directly involved more than 1 00 million people from over 30 countries . In a state of total war , the major participants threw their entire economic , industrial , and scientific capabilities behind the war effort , erasing the distinction between civilian and military resources . World War II was the deadliest conflict in human history , marked by 50 million to 85 million fatalities , most of which were civilians in the Soviet Union and China . It included massacres , the deliberate genocide of the Holocaust , strategic bombing , starvation , disease and the first use of nuclear weapons in history .[ 1 ][ 2 ][ 3 ][ 4 ]