This may not be a direct answer to the question, but its important to remember the context of 1939.

The UK and France greatly feared causing a wider, global conflict through aggressive foreign policy and military force, and for good reason, as the global situation was definitely not moving in their favour. Lets consider the world of 1939:

• Europe has three expansionist powers actively challenging the status quo. Germany has absorbed most of central Europe, including a key ally of France, Czechoslovakia. Russia trying to expand into Finland, and the fear of communism in the west at this stage was VERY real. Italy, having taken Ethiopia, was threatening the Balkans and eastern Mediterranean, with a modern (at least in terms of hardware) navy, who were a very real threat to British sea routes through the Med and towards Suez and Asia beyond. All three of these powers had directly interfered in the Spanish Civil war also, and as a result, France was now surrounded by hostile regimes.

• In the east. Japan was brutalising China and threatening French and British Holdings in Asia, such as Indo-china, Hong Kong, Singapore, and maybe even India and Australia.

• The USA seemed widely uninterested in engaging with Europe, as exemplified in their absence from the League of Nations, and their support in a global conflict was by no means guaranteed.

Combined with the fact that the UK and France were not prepared for such a conflict, and with the recent memory of what such a war would look like (WWI) it is not surprising that they were willing to forgo what would now appear to be decisive military action, for the sake of avoiding a war.

Mainly because the Allies weren't really mobilized, or prepared for War.

Aside from that, the entire French mentality was oriented towards being defensive - building their Maginot line, holding fast, and waiting for their allies to arrive in significant enough numbers - like in the last world war. It was perhaps too much to expect the French to alter their thinking, and strategies so much and in such a short period of time.

1. Poland fell far more quickly than anyone imagined, leading to world-wide shock and the coining of the term 'Blitzkrieg'.

2. France and England were both colonial empires with large overseas holdings. Compare this to Germany, which had territory exclusively in Europe, and a population almost twice as large as Metropolitan France's, and quite a bit larger than Great Britain's home population. So there is an argument to be made that the Allies needed time to mobilize not just their home front, but also their colonies.

3. I've heard it said that the Allies still hoped for a peaceful solution, or expected Hitler to back down and worried that a full-scale assault on Germany would have negated such an opportunity.

In the end, why the Allies didn't attack Germany straight away is a subject of a lot of debate. Alfred Jodl admitted at Nuremberg that an attack on Germany while the majority of the Wehrmacht was tied down in Poland would have been disastrous for Germany and likely would've ended the war. My personal opinion is that the Allied leadership suffered from a great deal of wishful thinking, and misunderstanding of Hitler and his motives, and were always too slow to act...but of course we say that now, with perfect hindsight. Nobody then could've predicted the holocaust, or reasonably guessed at Hitler's personality and mania.

One last point I want to make, in defense of Chamberlain (who is likely going to be blamed and derided a lot). At this point in its history, the British Empire was already beginning to fray, and there was a fear that the Empire wasn't durable enough to withstand another large war. To the view of Chamberlain, and many British, losing/winning a war wasn't as important as the maintenance of the Empire, and to that point look what happened - a Briton teleported from 1939 into the year 1960 would have had some difficulty deciding who had won World War 2. India, Egypt, and much of Africa had left the Empire at this point. So the question is, was victory over Nazi Germany really a victory for the British Empire?

Britain didn't want a war, it wanted time to consolidate, and get its house in order. Participation in the war led to a weakening of the Empire to such an extent that the rumbles for independence throughout the colonies finally reached a tipping point, and it all came falling down in the space of a decade.

Edit: For a book that gives a comprehensive account of the war, I'd recommend The Second World War by John Keegan. ( http://www.amazon.com/The-Second-World-John-Keegan/dp/0143035738 ) It can be a little hard to read at times because it throws a lot of facts and figures at you, but I found it to be extremely informative, and it goes into depth on most subjects of the war.

I'd like to add to /u/Raventhefuhrer/ 's answer (hi Raven!).

The strategic aim of the Maginot line was to render that part of the French border immune to attack, and thus force Germany to invade through a limited corridor, namely Belgium. You can read more fully here, here, and here.

To give a fuller context, the Franco-Prussian war of 1870 was fought along the modern French-German border, ending in strategic defeat of France as Prussia was able mobilize its troops much faster than France did, and thus were able to annihilate French formations through encirclement. Germany tried a similar approach in WW1, except that they went through Belgium, but this that time, surprise, the French were able to mobilize, fight an effective retreat, and stabilize their lines instead of being encircled.

So the experience of the Franco-Prussian War, and WW1, weighed heavily on French military planners leading up to WW2. Further, the western front of WW1 was fought almost entirely in France, in a heavily industrialized area. This meant perceived disadvantage in armament and manpower. So the Maginot line was an effort to limit the frontage, and thus economize French resources.

A large chunk of British Empire forces were spread across the globe policing the Empire and while there were plans in place for a British Expeditionary Force in to be deployed in France, the forces in Britain were not fully mobilised, nor had rearmament been completed. Nonetheless British forces began deploying to France in September 1939, but the first deployment was not completed until early October, by which point the Polish campaign was over. In addition the first deployment was only 158,000 men, and there were severe deficiencies in equipment. There was no practical way for Britain to launch any kind of offensive to help the Poles.

As for the French, they had manpower shortages and did not want to suffer a repeat of the massive losses they had taken in WW1, so they planned for a defensive war. Even if they had continued offensive operations against the Germans, it is highly debatable that they would have got anywhere. While on paper the French Army was well equipped, it lacked key equipment such as radios, training in combined arms warfare, and much France's high command and General Staff lacked the mindset to carry out aggressive offensives.

Britain and France probably should have been much clearer to Poland about the state of their forces in 1939, as neither country was in a good position to start offensive operations in support of Poland.

The French spent vast amounts of manpower and materials, building the maginot line. I believe their army was also trained mostly in static defense tactics from world war I. Their tank force for example, which was one of the largest in the world when the war broke out, were used as infantry support rather than grouped weapon systems. They would spread armor evenly along infantry lines instead of of concentrating them together to punch holes into enemy lines. The French knew that their forces were not trained for an offensive war into Germany and therefore stuck to their static defense tactics. For the British I just don't think it was feasible for them to mobilize a sufficient force in time. Chamberlain believed the Munich Agreement guaranteed peace in Europe. So when a year later Hitler invaded Poland, England still had to conscript soldiers, train them, arm them, feed them, supply them, transport them, which would take time.