UK was allied USSR.
The 1939 invasion of Poland caused Britain to declare war on Germany, which started WWII.
Because Germany invaded Poland.
Germany attacked a Poland A British ally.
Because of Germany's attack on Poland.
The event was Germany invading Poland.
Because at 1 September 1939 Germany invaded Poland.
the invasion of Poland in Sept 1939
I presume you are talking about WW2. Britain & France had a treaty with Poland to ensure Poland's safety and when Germany invaded Poland in 1939 this treaty was violated and therefore Britain & France had no option but to go to Poland's aid
They said that they would declare war on Germany
Germany started a series of invasions of neighboring Countries. France and England had a treaty with Poland to mutually defend each other. When Germany invaded Poland It was ON!
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.
It caused Great Britain and France to declare war on Germany.