However, Hitler would have never invaded Poland had the Soviet Union (i.e., Joseph Stalin) provided the latter with guarantees similar to the one provided by Britain and France.
For the one simple reason – in 1939, Nazi Germany did not have the resources to fight the war with the Soviet Union alone (let alone war on two fronts with the coalition of the Soviet Union, Britain, France, Canada, Australia and French and British colonies).
Unfortunately to “all of the above”, Joseph Stalin had no desire to prevent World War II. He had the entirely opposite desire – to start the war, let Germany, France and Britain bleed each other in a prolonged conflict a-la the First Great War and then attack them at the most convenient time, defeating, conquering and occupying these nations, the whole Europe and subsequently – the whole world.
So instead of providing Poland with military guarantees, Joseph Stalin did exactly the opposite – gave Adolf Hitler a green light for invading Poland by signing (on August 23rd, 1939) the secret protocols to the infamous Molotov-Ribbentrop pact committing to invasion, occupation and annexation of Eastern Poland thus dividing Poland with Nazi Germany.
Consequently, the Soviet Union (and personally Joseph Stalin) share the blame for the outbreak the Second World War with Nazi Germany, Britain, France… and with the United States of America.
Great Britain is an island. Consequently, it is critically dependent on supplies from other nations – especially the United States. To put it bluntly, there was no way for His Majesty Government to fight the war with Nazi Germany without receiving crucial materials from the United States.
While the overwhelming majority of the American population was thoroughly isolationist, that was not the case with President Roosevelt, his administration and a number of key figures in the ruling elites of the USA.
Contrary to the popular misconception, in 1939 the Roosevelt administration was still not very successful in their battle with the Great Depression. And much less successful than the Nazi government.
Consequently, it desperately needed a major war in Europe (ideally, the global war) which the United States will enter at the most convenient time (as it did in the previous Great War).
The war that will provide the USA with an attractive foreign buyer of its military equipment and non-military materials and will subsequently allow the federal government to radically increase its military spending (and thus to finally pull the country out of the Great Depression).
All President Roosevelt had to do to prevent World War II was to openly warn Britain that it will provide it with the necessary supplies only if a foreign power declares war on Britain. And if Britain declares war on any foreign power (for any reason), the United States will immediately introduce a strict embargo on all shipments to the United Kingdom.
But (for the abovementioned reasons) President Roosevelt did exactly the opposite. He assured the British government that he will find the way (despite the Neutrality Act then in force) to supply Britain with both military and non-military materials and equipment should Britain enter the war with Germany. Regardless of who declared war on whom.
And that’s exactly what he did by first instituting so-called “cash-and-carry policy” (on November 5th, 1939) and on March 11th, 1941 replacing it with the now-famous lend-lease program. Which to a very significant extent won the war for the Allied powers.
Therefore, contrary to an, alas, very popular misconception, Nazi Germany does not bear sole responsibility for the outbreak of the Second World War. But shares this responsibility with the Soviet Union, Britain, France and the United States (roughly in that order).