That is debatable for several reasons. First why than did the allies do absolutely nothing to try and actually help Poland. Surely some sort of offensive while the Germans are distracted could have if not worked at least shown initiative. And secondly the Germans did not attack Poland alone. So why declare war on them and not say the Soviet Union? Sure Poland was a good excuse but was it the true motivation of the political leadership?
The French did make an offensive, they quickly called it off because they had too many problems and not nearly enough troops to expect to beat the Germans. Poland was collapsing too quickly, and more German troops would have swiftly been moved west. The German army outnumbered the largest possible French force about 2:1 overall. Poland might have lasted longer, but only had she fully mobilized, as it was political considerations led to a delayed mobilization and only about half the Polish army was in the field on September 1st 1939; though it had more then half the weapons on hand. A major allied effort was made to send aircraft and weapons through Romania, more then a few aircraft got to the country but very few had been assembled and delivered by the time the nation capitulated. The reality was Poland collapsed too quickly to be helped, and the French, who could not have been supported by more then a few British divisions, quickly became concerned that the Germans would immediately shift forces west. The available allied force in the west in the fall of 1939 was simply not large enough to defeat the Germans. Maybe 40 divisions
As for the USSR, the allies had some idea that Stalin was not interested in full on war with them, and knew he had only attacked when Poland was already clearly defeated, so they were not about to rush into such a conflict. But in fact, once Stalin attacked Finland the British and French made plans to send troops to aid the country, and did send considerable war material. The Sweds refused to allow transit of actual troops to fight, and that directly led into the plan to invade Norway. A plan was also hatched, and the groundwork laid for a bombing campaign against Baku to cripple the Soviet economy and hopefully Germany by cutting off the oil flow. This plainly would have meant full scale war between the allies and the Soviets.
Mr Bean wrote:Followup question to Sea Skimmer, what exactly did Hitler offer in 1940 when Britain stood alone. It was my impression that he offered very little to the British and that in 1940 he was less interested in dealing than accepting their surrender.
It was hinted to the British that certain European states might be restored to sovereignty, but only under unquestioned German domination of the entire continent, that Czechoslovakia would not be one such state, that Dutch and French colonies would be open to negotiation, implying that Hitler expected to annex at least some of these territories and perhaps reward Italy as well. About the only thing firm was that the Germans would demand nothing from Britain possessions. This was basically conveyed through Sweden, direct German approaches via Switzerland basically just asked the British to come to terms. It was also hinted that the British would need to agree to some kind of ceasefire before full scale negotiations could take place. So not exactly a demand for surrender, not of Britain, but certainly a demand to surrender Europe to Hitler.
Some in the British government were still in favor of it in June 1940, believing the war already lost and any terms better then invasion, with Lord Halifax at the head of this moment. He was defeated swiftly by Churchill. Once the British got past the initial invasion scare another time never existed in which surrender or peace was credible. It would have taken starvation by U-boat blockade to do that. By September 1941 when the US took the gloves off on neutrality law, USN aircraft carriers, not just a few destroyers or what have you, were actually escorting convoys as close as 700 miles to Brest, they broke off only at the edge of German bomber range! That's more then two thirds the way across the Atlantic Ocean.
"This cult of special forces is as sensible as to form a Royal Corps of Tree Climbers and say that no soldier who does not wear its green hat with a bunch of oak leaves stuck in it should be expected to climb a tree"
— Field Marshal William Slim 1956