England and France warned Germany that any invasion of Poland would result in war. When Germany attacked Poland France and England honored their treaty obligations with Poland and attacked Germany.
In WW2 the French and Britain said they would declare war on Germany if they attacked Poland. When Nazi Germany invaded Poland France and Britain declared war on Germany. Then Germany stormed French.
Britain and France had offered a guarantee to Poland that if she was attacked by Germany they would come to her aid. When Germany subsequently invaded Poland, Britain and France gave Germany an ultimatum, demanding the withdrawal of German troops from Poland. Failure to do so would result in a state of war.
The British were a part of the Anglo-Polish military alliance, an agreement between several countries, including England, France, and Poland. When the Germans invaded Poland in the fall of 1939, Great Britain gave the Germans an ultimatum to pull all of their troops out of Poland immediately, but Germany ignored them. A day later, Great Britain and France declared war on Germany, as they had agreed to do as part of the military alliance.
On September 1, 1939 Germany invaded Poland. Great Britain and France had warned Hitler that if he invaded Poland, they would declare war.
4 September 1939 because New Zealand was one of the first countries to be involved in the global conflict precipitated by Germany's invasion of Poland on 1 Sept. It was a consequence of Britain's defence pact with Poland, and consequent war declaration on the 3 Sept, and many British commonwealth countries were of course directly affected.
Nazi Germany entered Poland on September 2, 1939. Prime-minister Nevil Chamberlain issued an ultimatum to Adolf Hitler that if Poland were attacked, Great Britain would come to her aid.
Germany
In both cases, because Germany attacked countries with which the UK had alliances or to whom she had given guarantees. In 1914, when Germany attacked France via Belgium, Britain, as a member of the Triple Entente (Britain, France and Russia), was obliged to go to the defense of France. In March 1939, after Hitler had broken the 1938 Munich Agreement and occupied Czechoslovakia, Britain and France gave guarantees to Poland that they would go to war with Germany if Hitler attacked Poland, which, of course, happened on September 1, 1939. After Germany ignored Britain's ultimatum to withdraw from Poland, she declared war on Germany on September 3.
On 1 September 1939, Germany invaded Poland without a declaration of war. Britain and France had a defence agreement with Poland which stated that they would come to their aid if they were attacked. Britain and France then sent an ultimatum to Germany demanding that their troops withdraw from Poland. Germany ignored the ultimatum so on 3 September 1939, Britain and France declared war on Germany.
It is completely irrelevant as Germany would have attacked Poland anyway.
In late August of 1939, Germany and the USSR signed their German-Soviet Non-aggression Pact. The public treaty had a secret clause. It allowed the Soviets to take control of the eastern half of Poland if Germany attacked Poland. This agreement insured for both Stalin and Hitler, they would not be adversariesIt allowed Hitler to use as many troops he believed was needed for the fast invasion and conquest of Poland.