When Russia invaded Poland to assist Germany in 1939 , I wonder what would of happened if England and France Declared war on them also?

Well several things might have occurerd.

1. France and England were in no shape to actually go to war with Russia, so the outcome might have very well been the same scenerio which Germany experienced with France before she invaded......

2. Russia, on the other hand would have had plenty of time to mobilize for war, which she had not done at the time of Operation Barbarossa.

3. Germany might not have attacked Russia, instead moving its 3 million troops to other places like Africa and secure the oil which Germany desperately needed.

4. Barbarossa was a pretty big reason why operation Sea Lion did not occur. Had this been the case, Great Britain might have very well found her self in a bit of trouble.

There are ofcourse many and many outcomes which could have accured none if favor of the Allies. Even an alliance between Germany and Russia might have been possible. One thing is for sure, this would have seriously complicated the situation in Europe and the world.

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The war against Russia will be such that it cannot be conducted in a knightly fashion. This struggle is one of ideologies and racial differences and will have to be conducted with unprecedented, unmerciful and unrelenting harshness. -Adolf Hitler

A good question would be why didn't Great Britain and France declare war on Russia since it too invaded Poland? But to the original question, had war been declared on Russia, it would have been recognized by all parties that the Russo-German alliance would have been an uneasy one. Hitler probably would have made heavy attempts after the fall of France to persuade Great Britain to ally itself with Germany against the Bolshevic state.

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American by birth, TEXAN by the grace of GOD!

My best guess is that in the end the Nordic countries would have ended as the battle field instead of continental Europe.

The allied would have been forced to prepare Norway and Sweden for an attack, and the USSR, if had invaded Finland, would have been a major threat to Kiruna iron fields, and both the allied and Germany would have been forced to act on this.

The same would have happened if the USSR had invaded the whole of Finland during Winter War. Norway and Sweden would have been invaded by friendly allied troops unless the USSR had continued the attack to Sweden.

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What could France and England have done with Germany in between? If worse came to worse an attempt of a offensive from France backed with English troops to Germany (very unlikely) could have consolidated the Russo-German Alliance and allowed Hitler to attack France and England with Russian support. Strategists must have considered this option and thought it was wise not to declare war on two mighty ennemies at the same time.

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IIRC, there were very serious discussions, among French military and political leader, to a plan against USSR, which was based for the most on heavy bombings of Caucasus oilfields.

LOL ! These guys were really flying high above reality !

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England and France were never strong allies during any time in history. Even in WWI they kept to their own parts of the battlefield. Countries with different languages usually don't make good allies in general.

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The alliance between Germany and Japan proves language is not always an issue and the Entente Cordiale alliance was strong and sincere during WWI. The Belgians also kept their own army and they spoke French. The French and the English have often fought as allies on the same battlefields (Crimea in the 19th century, WWI , Narvik, Suez etc.. ). I think Narvik is interesting because both Head Quarters must have imagined a stronghold in Norway that could have led to Russia later on.

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I don't think I'd point to the alliance between Germany and Japan as an example of a strong alliance despite language differences. These two nations were allied for no other reason that the old adage "the enemy of my enemy is my friend". Both regimes despised communism and were held in extreme suspicion by the Wester Powers. Their alliance was one of convenience and little more. The Germans, with their racial ideas, could hardly come to fully embrace an Asian empire as their equal and full partner. There was very little interaction between the two nations and no real coordination in terms of strategy or tactics.

As to what would have happened if the Western Allies had declared war on the Soviet Union in 1939, well, I think the governments of both France and Britain would have likely fallen. Taking on Germany was tough enough; with Russia added on the task would have seemed overwhelming and that would call into question the leadership. It would have been a very, very unpopular war. Britain would, in this scenario, have to significantly increase the ground forces it held in the Middle East and India if it did not want to lose these vital territories, which would now be under threat from Soviet forces to the north. As Kai-Petri has pointed out, a major theater of war would have likely been the Scandinavian countries and simple geography here would tend to favor the Germans and Russians, given how much closer they were to these areas. And if Britain already had to send off a lot of its reserves to reinforce the Middle East and India, how would they come up with a force to seriously threaten both Germany and the Soviet Union in Scandinavia? As far as I see it, this "What If?" scenario definitely favors the Germans.

How long would this alliance between Nazi Germany and the Soviet Union would last? I can't say but I imagine it would have lasted, as in the real world, just as long as the two countries faced immediate danger. Once France was out of the picture (and there is little doubt it would fall in this scenario) Germany would still be left unable to cross the Channel and so Germany would start looking east again. How long Hitler would be willing to postpone Barbarossa is unclear, but I imagine he would be less quick to attack Russia if it was aiding his cause by keeping the Brits occupied out east. However, he may well have taken the opportunity to stab Stalin in the back while the Red Army had fewer units guarding the western borders.

Would Stalin have even trusted Germany enough to attack Britain in India and the Middle East? Frankly, I doubt Britain could have mustered enough force to be a major threat to Siberia (and what would be the point of advancing into that huge, relatively unpopulated wilderness?) so I imagine the situation between Siberia and the Middle East and India would have been a sitzkrieg. I doubt there would be much action but in case there were both sides would have to station considerable forces in the threatened areas to act as a deterent. Hitler would still have to attack the Soviet Union at some point (because, being Hitler, that was a raison d'etre of his life) and when he did it would be interesting to see just how quickly Stalin would try and patch things up with the Allies. And, I doubt that the Allies would mind dropping the war against Russia in return for an ally against Germany. And so we're back to basically the historical scenario. All in my humble opinion, of course.