It was on March 31, 1939 that these two world powers at the time decided to take some measures against the Nazi Germany that was preparing for what was undoubtedly a siege if not a war against Poland, given all the previous events I already mentioned in previous posts.
The leaders of Poland understood and clearly knew they had no chance against a German invasion, so they had a meeting with the French and British leaders to sign an agreement. France even signed in a different agreement with Poland that, in the event of a German invasion to Poland, France would launch an defense in no later than 15 days after the invasion.
Of course this never happened, even though Britain and France did declare war after Germany invaded Poland, the French army only made a brief advance toward the Siegfried Line on Germany's western frontier and immediately stopped upon meeting German resistance.
This was probably the first event that really could've changed the tide of the war or prevented it from getting to a 6 year long war with tens of countries and millions of deaths.
You see, Germany had most of their military forces in the eastern frontier, and France had a big army. Had France not stopped when they met some little German resistance, just like they promised to Poland, they could've recovered Poland and made some serious damage to the German troops and their morale.
But instead of that, Germany was able to conquer Poland very easily and then with little to no resistance, mobilize their troops to the west.
The British and French betrayed Poland out of dishonesty, but also out of stupidity because they could've prevented the following events that devastated Europe.
A crowning humiliation of the Poles was the refusal of their British "friends" to allow the free Polish army to march in the victory parade at the end of the war for fear of offending a Soviet puppet government in Lublin.
Image is a meme, but it really portrays what must've been going through Hitler's mind when planning for the invasion of Poland.