Originally posted by Octavius
A small time frame did exist. France/GB had a much larger capacity to wage war. Their economy had much more depth than width (not a whole lot of bite, but an ability to weather the storm for a much longer period). Germany had more width (stockpiled weapons/resources for quick bursts of engagements). But that was only one reason.
As a general rule, going on the experience of the first world war, France and GB saw the attacking force would lose twice as many men/resources.. A defensive, passive stance for the allies would set the tone for the the early years of the war. This was coupled with the sheer unwillingness to fight. "An entire generation lost" 25~ years prior... it was still fresh in their minds.
[edit]: oy, like bike said, 1939 or 1944?[/edit]
The reluctance to fight had gone by '39. The problem was neither country was really in any state to wage war (especially Britain). The UK had only started re-armament slowly in 1938 after years of neglect. It is possible that swift action while the Wehrmacht was concentrating on Poland might have brought a swift peace but considering the disasters of the summer of 1940 I rather doubt anything could have been achieved a year earlier.
As to sending the Polish brigades to Warsaw the RAF and the USAAF didn't have the capacity to send such large forces by air over long distances (and through heavily defended air space). It might have been possible to airlift a few battalions by converting some of the large bombers but there wasn't time for it and it would have required the co-operation of the USSR which wasn't willing to countenance any such move.
Consider for a moment how close many of the Para drops over Normandy got to disaster: that was much closer to base (less than 200km), with much more ground support, over open countryside after months of planning and practice, and with much more reconnaisance.
What happened to Poland was terrible but there was no way for the Western Allies to prevent it. Only Russia could have done anything and it didn't want to. Agreeing to Russian dominance in Eastern Europe wasn't what either Chruchill or Roosevelt wanted it was just fait accompli: the Red Army was already there with millions of men under arms. The only alternative was war with Stalin which (if it had even been possible considering the strain of all those years of warfare had created) would have led to yet more years of bloody warfare.
Without Russian help the Warsaw Rising was doomed. Without the Germans there would have been no-one to rise against. But it's the Western Allies you are bitter with?