World War II officially began after Germany invaded Poland. The invasion of Poland caused England and France to declare war on Germany.
The 1939 invasion of Poland caused Britain to declare war on Germany, which started WWII.
Hitler lead Germany to invade many other countries in Europe. Since many of the countries in Europe were part of the Allies such as: Britian. Then they bombed Pearl Harbor in Hawiia which caused the U.S. to join the war.
The assassination of Austrian Archduke Ferdinand was the immediate cause of World War I. He was killed by a Serbian terrorist, which caused Austria to declare war on Serbia. Serbia was allied with Russia, who intervened, causing Germany to declare war on Russia, which caused France to declare war on Germany.
Poland
It caused Great Britain and France to declare war on Germany.
Belgium .
The Germans attacked Poland, this caused England and France to declare war on Germany, so Germany declared war on them.
Largely due to economic problems caused by the U.S. Great Depression.
Once Germany invaded Poland, it caused England and France to declare war on Germany. It was the official beginning of World War II.
Germany invaded Poland privacy and the allies France and Britain declared war
Simple answer, Germany invaded Poland in September 1939 which caused Britain and France to declare war on Germany. There you have it, start of WW2.