The Nazi-Soviet Pact left Poland isolated and surrounded. Orders were given for units of the German armed forces to attack Poland. To Hitler’s surprise the British had agreed a military alliance with the Poles on 25 August. This had caused Hitler to delay his plans to attack Poland.
In the meantime, attempts continued by the British and French to avoid the outbreak of a war. These failed.
On September 1st the Germany attacked Poland. Both the British and French gave an ultimatum to Hitler demanding the withdrawal of German troops from Poland.
On September 3rd the British declared war on Germany.
Some historians believe that this came as a surprise to Hitler. He did not believe that the British would go to war over Poland. They had failed to support Czechoslovakia in 1938.
Hitler also believed that he would be able to negotiate with the British. He thought that the desire to avoid war would mean the British would allow Hitler a free hand in Europe. In return they would be allowed to keep their Empire.