They abandoned Poland at the end of the war when they allowed the Soviets to include Poland and the rest of Eastern Europe into its sphere of influence. They did not abandon Poland at the beginning of WWII.
They declared war in name -- that's what it was called from 39 to 40 as the Phony war. If the French and English had actually attacked Germany instead of just talking, they would have ended the war in 1939 and saved millions of lives -- a lot of it their own.
Neither Germany or France and England took offensive action on the Western Front save for the French Saar Offensive that went into Germany a few miles on September 7. Hindsight is 20/20 as to whether the French and English could have invaded Germany successfully despite outnumbering them in forces. England was still unprepared for war and the French favored a defensive strategy.
That said, there is no justification to say that France and England abandoned Poland. They declared war on Germany with all the consequences that entailed for their countries. It was a serious commitment in support of Poland and against German aggression. The German and Soviet invasions of Poland and subsequent surrender of Poland in four weeks made it very difficult to intervene. Let's look at the chronology encompassing the "phoney war:"
1 September 1939
"Operation Weiss": German troops invade Poland. German tanks thundered across the Polish border at precisely 0445 hours, supported by Junkers Ju87 ("Stuka" named Sturzkampfflugzeug) dive-bombers.
At 9.00 am England and France issue an ultimatum to Germany.
2 September 1939
The ocean liner S.S. Athenia is torpedoed by the German U-boat U30. Over 112 civilians died at sea, 69 women, 16 children and 28 men. America and Canada were popular destinations, trying to escape from War.
7 September 1939,
French troops cross the German border at the "Saar-front." Eleven French divisions advanced along a 32 km (20 miles) line near Saarbrücken against weak German opposition. The attack did not result in the diversion of any German troops.
17 September 1939,
Russian troops invade eastern Poland.
The British aircraft carrier HMS Courageous was sunk by U-29. She went down in 15 minutes with the loss of 518 of her crew, including her captain. She was the first British warship to be lost in the war.
19 September 1939,
Soviets and Germany meet at Brest-Litovsk.
24 September 1939,
Start of the bombing of Warsaw with 1150 bombers. Warsaw is blitzed from the air as the German ground forces prepare for an assault on the capital.
27 September 1939,
Poland surrenders to Nazi Germany
29 September 1939
Germany-Russia treaty of frontier- and friendship at Moscow, providing for division of Poland. Esthonia signs mutual assistance pact with Soviet Union.
6 October 1939,
The last Polish ground troops surrender. Polish national losses: 700,000 men captured by the Germans, 200,000 men by the Russians and great loss of life; more than 120,000 troops could escape. The Germans lost 10,000 men and Russia several hundred.
In a major speech to the Reichstag, Hitler speaks of his desire for peace with Britain and France. Hitler says that up to now he has done nothing more than correct the unjust Versailles Treaty and that he has no war aims against France or Britain. He blames warmongers like Churchill for the present state of affairs and calls for a European conference to meet and resolve the few remaining differences.
Hitler wanted revenge on France, and it was going to happen sooner or later.
But I do believe he still wanted peace with Britain, that’s why IMHO, he sent Hess over there. Hess would have NEVER disobeyed Hitler, he was as loyal as it gets.
They could have prevented the Battle of France and Battle of Britain