I hate to say it, but I doubt France and Britian could have stopped Germany once the war started, no matter what really. The Germans possessed better tanks, planes, soldiers, rifles, pistols, artillery, training, Generals, central command, intellegince, logistics, scienctists, R & D, roads, industrial capacity, drive and belief. They also created that type of warfare that the allies copied in order to be affective. Heavy bombing before rushing in with the best tanks around and very well trained soldiers who were swift and mobilized, France and Britian stood no chance.
By the end of the Battle of Flanders, Britian France and Belgium had been humiliated and evacuated 338,226 troops from Dunkirk, I don't have the figures for how many more soldiers were lost.
As far as Germany invading Russia, lets take a look at what caused German high command believing they could defeat the Soviets. In 1939, Russia invaded Finland. Of course, anyone familiar with Russian military history knows Russia's classic (yet absurd) attack strategy. That is the Russion Onslaught, attack the objective with every
available man on every open avenue no matter the losses for either side. Tiny Finland, was very limited in man power (a population at the time of about 7 million, but I think I'm going over a million) and had WW1 era hardware. However, they had the will to fight and knew how the Russians would attack. The result, of course, was a Russian victory. However, if you look at the casualties, it may not seem apparently so.
Finland Losses - 25,000 killed or missing. Another 43,000 wounded.
Russian Losses - Giving conservative figures, atleast 200,000 dead (many say it goes higher, as the Russians had no problem feeding Finnish kill boxes, which were areas of extremely high concentrated artillery fire). Atleast another 400,000 wounded, but may go as high as double that.
In the battle of Suomussalmi, the Russians started with a 3:1 strength superiority. When the battle was over, the Finns lost 900, 1,700 wounded. The Russians lost 27,500, I could only imagine how many wounded as the figure is not listed. That was just one battle.
Ultimately, the Germans felt that they had across the board superioty over the Russians, except total manpower. The fact is, they did. The biggest blunder here is, and I believe another poster said it, was that Hitler took control of German command shortly after the Russian invasion. His Generals up to that point operated with remarkable freedom and it was quite revolutionary. They were given objectives, but it was up to them to decide how the operations were carried out. This led to decisive victory after decisive victory and showed just how brilliant the German generals were. However, Hitler took control, made stupid decisions, had ridiculous objectives, did not listen to his Generals, and severly hampered the German armie's ability to fight. Russia was well on its way to defeat, they had suffered losses of over 20,000,000 at the hands of the Germans (Fighting, bombing, starvation, extermination), and over 25,000,000 by the wars end. They survived because U.S aid, the Russian Winter, and Hitler's own foolishness and ego (HE wanted to be the one who beat the Russians).