British intelligence conducts a wide-ranging independent assessment of German strength instead of relying totally on French reports as they did in OTL. As a result they realise they can defeat the Germans if it comes to the latter invading Poland. Urgent communications begin between Paris and London with the French agreeing to an offensive move by the Allies if Britain will support the French attempts at collective security including their alliance with the USSR that had recently been scuttled by the UK.
Litvinov is rehabilitated as serious high-level discussions begin between the allies and Stalin. An agreement is reached whereby the USSR will guarantee the national security of Poland whether the Poles agree to Red Army troops on their soil or not. The agreement is vague about Soviet claims on Polish territory and the unwritten understanding is that the USSR will be permitted to keep some of the Polish lands bordering them. Poland is of course unaware of this understanding. Britain pressures the Poles to agree to allowing Soviet troops on their territory by unconditionally supporting Polish security and national integrity.
Hitler must proceed with his invasion but without the non-aggression pact with the USSR. This means a longer and more costly offensive. As more German troops are drawn deeper into Polish territory, the French and British launch a determined and heavy attack in the west. It unfolds just like the Saar offensive in OTL except there is no fear of encountering superior German forces. Pressing the attack, the allies overrun and occupy the industrial heartland of Germany causing the Germans to begin preparations to move divisions from the east to launch a counterattack. The Polish government meanwhile flees into exile. As the German divisions are being readied to move west the Red Army launches a massive attack in conjunction with some remaining Polish forces that have fled over the border and have been resupplied by the Soviets. Caught unawares the Germans suffer significant casualties and their timetable is disrupted.
Pressed hard by the simultaneous offensives from both east and west the German forces begin to waver. Munitions and vital supplies are running out as the shipments from the USSR have ceased for some time. Facing the nightmare of a two front war that they seem to be losing, the German military elite stages a coup and replace the Nazis with a junta. They sue for peace and the terms are relatively mild but include the large reduction of the German military machine and a civilian government to replace the junta through elections and the acceptance of a small, token occupying force.
After signing the peace, Stalin replaces Litvinov again and refuses to retreat from all Polish territory as was his understanding. Despite the fact he has agreed to the reestablishment of the Polish state he refuses to surrender territory he claims is a security buffer and was promised to him. Although the amount of land is relatively small, the allies demand he surrender all of it to the soon to be re-established Polish state. As a result of continued Soviet occupation, the German government does not withdraw its forces from Poland. The allies agree to this ‘temporary’ measure despite the Polish government in exile making loud and vigorous complaints.
Stalin’s refusal to withdraw leads to war and both Germany and Italy agree to send contingents to assist the allies. Spain soon follows suit, as does Portugal although both only send small volunteer units.
It soon become clear the conflict will be long and bloody and public opinion is not in favour of a war over what many perceive to be a minor readjustment of the borders. Indeed, many in the west see the Soviet claim as justified and after all the Germans who invaded the place are being allowed to continue to occupy it by the allies. When news leaks out about the secret deal the subsequent outrage forces the allies to seek a negotiated peace with the Soviet Union. Part of the agreement ensures German forces are kept low but Germany is permitted to occupy the Polish territory it conquered in 1939. This is justified as a security measure against Soviet aggression. The German government agrees to allow its small military to act as a reserve force for the allied armies along the border with the Soviet Union and to pay for the allied occupation. The Soviets are permitted to continue their occupation of the small strip of Polish territory but must agree to demilitarise it.