Andrew Roberts the author of the book, The Storm of War: A New History of the Second World War, says that Britain and France could have attacked Germany and defeated it when the cream of the Wehrmacht was busy in Poland
The fear of a war on two fronts led the Führer to detail no fewer than 40 divisions to protect his back. Crucially, however, three-quarters of these were only second-rate units and they were left with only three days' ammunition. His best troops, along with all of his armoured and mobile divisions and almost all his warplanes, were devoted to the attack on Poland.
The Americans cross the Siegfried-Maginot Line later in the war. If only the British and French had done that when Hitler attacked Poland........
Says Roberts in article in The Telegraph...
Hitler was forced to leave 40% of his 100-division army out in the West, guarding the still-incomplete Siegfried Line, or 'West Wall'. The fear of a war on two fronts led the Führer to detail no fewer than 40 divisions to protect his back. Crucially, however, three-quarters of these were only second-rate units and they were left with only three days' ammunition. His best troops, along with all of his armoured and mobile divisions and almost all his warplanes, were devoted to the attack on Poland.THE NAZI LEADERS WERE SCARED!
Had a large, modern British Army and RAF been waiting up against the Siegfried Line in late August 1939, primed for action and superbly armed, trained, equipped and led, having been properly financed in the two decades since the Great War, and ranged alongside the French army to invade Nazi Germany the moment the Wehrmacht crossed the Polish border, history could have turned out very differently.
Paul Schmidt was a translator in the German Foreign Ministry and present at the history-making events of those last days of peace in Europe. The scene is the office of the German Foreign Ministry in Berlin. It is just after midnight on September 3, 1939 and the German juggernaut continues to slam its way into Poland. The Germans have not responded to an earlier British and French demand to withdraw their troops and a message is received stating that Sir Neville Henderson, the British Ambassador to Germany, wishes to meet with German Foreign Minister Ribbontrop. It is obvious to all that the Ambassador's message will probably mean war.
When I entered the next room Hitler was sitting at his desk and Ribbentrop stood by the window. Both looked up expectantly as I came in. I stopped at some distance from Hitler's desk, and then slowly translated the British Government's ultimatum. When I finished, there was complete silence.
Hitler sat immobile, gazing before him. He was not at a loss, as was afterwards stated, nor did he rage as others allege. He sat completely silent and unmoving.
After an interval which seemed an age, he turned to Ribbentrop, who had remained standing by the window. 'What now?' asked Hitler with a savage look, as though implying that his Foreign Minister had misled him about England's probable reaction. Ribbentrop answered quietly: 'I assume that the French will hand in a similar ultimatum within the hour.'
As my duty was now performed, I withdrew. To those in the anteroom pressing round me I said: 'The English have just handed us an ultimatum. In two hours a state of war will exist between England and Germany.' In the anteroom, too, this news was followed by complete silence.
Goering turned to me and said: 'If we lose this war, then God have mercy on us!' Goebbels stood in a corner, downcast and self-absorbed. Everywhere in the room I saw looks of grave concern, even amongst the lesser Party people."
Schmidt, Paul, Hitler's Interpreter (1951); Shirer, William, The Rise and Fall of the Third Reich (1960); Taylor, A.J.P., The Origins of the Second World War (1962).