Due to massive diplomatic efforts made by Halifax, Molotov etc, the UK, France and USSR sign an agreement in Aug 1939 guaranteeing the sovreignty of any nation attacked by Germany in Europe.

This is what all sides wanted and couldnt get because of lack of trust. the lack of any such agreement forced Stalin into the Molotov Ribbentrop pact, a morally reprehensible decision but, paradoxically, one Stalin had little choice but to take.

How does Hitler react when he knows any further aggression will be met by immediate French, Soviet and British assault?

Like in he did in the Sudetan, and then the rest of the country.
He didn't listen to his generals anyway. Hitler always backed himself.
He didn't think Britain and France would declare war when he invaded Poland.

Like in he did in the Sudetan, and then the rest of the country.
He didn't listen to his generals anyway. Hitler always backed himself.
He didn't think Britain and France would declare war when he invaded Poland.

no, but he knew even if they did they couldnt lend any practical assistance to Poland and it would be a while before they could mo9unt a serious offensive against Germany. but if he thought the Red Army would be attacking form the east, woul;d have have attacked Poland at all?

The Red Army wasn't that much of a threat in '39 either.

The Red Army wasn't that much of a threat in '39 either.

It was, despite incompetent leadership. Its mere existence - and huge size - guaranteed that. And the fact that the USSR would cut the supplies of raw materials to Germany immediately was an equally great threat by itself. Germany would be forced to quickly take all of Poland and then continue eastward, just to defend Eastern Prussia. Right in late Autumn and through Winter. It would also have to divert forces to defend Romania and its oil fields from certain soviet attack.

Could war start anyway and Germany do all this in 1939? Yes, but to have any hope of success they would have to strip all of their air force and almost all of its army from the western front. If the french and the british had any intelligence they'd cross Belgium and attack the Rurh then, and then proceed down the Rhine. The germans would have to either strip their eastern front before defeating the soviets (and lose Eastern Prussia, and perhaps even Romania's support), or lose its most industrialized region and its (poor) border defenses there. Both were politically sensitive territories: Prussia to the german officers, the Ruhr because of Versailles. Hitler and the Nazis would not survive such a blow. WW2 would not even become a Would War and have a diplomatic settlement by 1940, with a new german military government, with the Nazis blamed by the whole fiasco and suppressed by the military there.

Then again, the germans showed themselves, time and again, not to have any spine when it came to facing the Nazis...

... except in Asia.

Well the Soviets realy did want a piece of Poland, if not only because of traditional dislike and the expressed goal of spreading communism by any means, then at least because Stalin personaly got his proverbial ass whooped in the 1920 war and was after some payback.

So they would never have helped Poland without gaining something for them selves, and the Ribbentrop-Molotov pact was the easiest way to get it. They where "losely" allied with Germany.

The war in this case would be much easier for allies, especially for USSR. Hitler would probably attack Poland anyway, then after France, Britain and USSR declared war, on western front would be the same "phoney war". Stalin may use this possibility to attack trying to retake territories captured by Poland in 1920. Germans could not concentrate such forces on eastern front as they had in reality.

@innonimatu: you forget that Belgium and Romania were at first neutral in WW2 (and would most probably also be in this scenario)
taking this into account i think the Germans would be at least able to hold the Soviets off. at any rate i doubt isolationist Russia would be willing to enter into any form of binding pact with the west, at least not without their own ambitions in mind...

Stalin was terrified of war with Germany. No way he'd commit to such a war just to help the Poles- who he hated or Britain and France- who hated him.

The Western Allies sat around in France for a good 9 months and probably would have sat there for another 9.

Stalin was terrified of war with Germany. No way he'd commit to such a war just to help the Poles- who he hated or Britain and France- who hated him.

The Western Allies sat around in France for a good 9 months and probably would have sat there for another 9.

Actually him and Molotov were actively pursuing just this until it became clear the UK wouldnt have it

The UK and France were pushing for it, but it probably would have taken quite a bit for the Soviets to join them.
The Soviets simply gained more by a temporary agreement with Hitler than the west could ever offer. Stalin knew the USSR was not ready for a war against Germany, and the Molotov-Ribbentrop pact gave the Soviets free reign to take their share of the East, and no powers to oppose them. It also provided them time to build their industry and army for the upcoming war (which the Soviets were planning for).

Actually him and Molotov were actively pursuing just this until it became clear the UK wouldnt have it

Due to the fact that Stalin realised the massive effort the USSR would have to put in to fight any war with Germany compared to Britain, he was not prepared to commit.

I've heard talk of Soviet plans to attack Germany but have never seen any concrete proof.

There would still have been a war, Hitler was just that kinda guy.

The real question is whether France and England would have actually done anything, even with a war on. When war did break out over Poland in '39, England and France did a whole lot of nothing, which is exactly what Hitler was counting on. There were almost no German forces in place to withstand an invasion, and the rest of the Wehrmacht was tied down working out the kinks of Blitzkrieg operations.

Had England and France had the foresight to seize the industrialized Rhineland while they could, Germany would have found herself in a tough spot. Would the presence of the USSR in the war have done this? Maybe. I have my doubts about whether Stalin would have ordered an advance in Poland/Germany immediately, instead waiting till the Germans had spent themselves on the Poles, and to make sure the Western Allies would actually do something.

My personal opinion is that neither the USSR or Western Allies would have done anything until Germany moved against one side or the other, at which point whoever was free would begin advancing towards Berlin. Unless there was another conference between the Allies agreeing to attack together. A big change would have been the absence of an invasion of Denmark and Norway; I don't think Hitler would have been able to spare the troops. This would leave Sweden exposed and neutral as well, and Germany without most of the resources she required, and the war likely done by '42.