Britain did not fight alone, the war also involved many countries. World War II involved 61 countries with 1.7 billion people (three quarters of the world's population).
Fifty million people lost their lives and hundreds of millions people were injured.
After World War One ended in 1918, Germany had to give up land and was banned from having armed forces.
In 1933 the German people voted for a leader named Adolf Hitler, who led a political party in Germany called the National Socialists or Nazis. Hitler promised to make his country great again and quickly began to arm Germany again and to seize land from other countries.
Shortly before 5am on Friday 1st September, 1939, German forces stormed the Polish frontier. Tanks and motorised troops raced into the country over ground, supported by Stuka dive bombers overhead. A total of 1.25 million Germans soldiers swept into Poland
Click on the play button below to hear Chamberlain's speech (now the full speech)
The Second World War was started by Germany in an unprovoked attack on Poland. Britain and France declared war on Germany after Hitler had refused to abort his invasion of Poland.
What countries fought on which side? (Axis and Allies)
Who were the main leaders in World War Two?
How did people protect themselves during the war?
Effects of war