You don't give a timeframe. Poland has been split several times ... Perhaps you are thinking of the Ribbentrop-Molotov Pact of 1939.
When Hitler invaded Poland and other countries in 1939 and 1940 he violated the Munich Agreement.
Poland
The Molotov-Ribbentrop Pact .
Poland was invaded & occupied by both Germany & the Soviet Union.
The Molotov-Ribbentrop Pact of 1939.
The Munich Agreement in effect tranferred the Sudentenland from Czechoslovakia to Germany.
After the agreement was signed Hitler ignored the agreement and invaded the rest of Czechoslavakia, then on September 1st 1939 he invaded poland.
Britain and France both had a defence agreement with Poland. When Germany attacked Poland on 1st September 1939, Britain and France both declared war on Germany two days later but their actions did absolutely nothing to help Poland.
In the 1930's, Britain and France had a defence agreement with Poland. On 1 September 1939, German forces attacked Poland, so on 3 September 1939 Britain and France declared war on Germany.
The United Kingdom (which includes England) and France had a defence agreement with Poland which stated that if any of them was attacked, the others would come to their aid. On 1st September 1939, German troops invaded Poland. On 3rd September 1939, the UK and France both declared war on Germany.
Britain (which includes England) and France had a defence agreement with Poland. Germany invaded Poland on 1st September 1939, so on 3rd September 1939, Britain and France declared war on Germany.