The smell of it was in the air, of an imminent German attack on Poland. It was because of this that Chamberlain and Halifax decided not to delay any type of action against Germany. On March 31st 1939, Chamberlain announced in a crowded House of Commons that if Poland was in any type of a threat of invasion, Britain and France would come to their aid. An Anglo-Polish treaty of Alliance was under preparation and Britain also included Rumania, Greece and Turkey under their protection. Though angered and surprised, Hitler was not deterred by this treaty. He commanded his generals to ready their troops and prepare for an attack no later than September 1st. Hitler's aim was to start "a war with sudden, heavy blows and gaining rapid successes." (Bullock-'91)
Hitler now depended on the Triple Alliance of Germany, Japan and Italy to be against Britain, America and Soviet Union. But Japan was not in favor of this Alliance because it was not ready for any confrontation from American and the British Navy. Since Japan was reluctant to be in the Alliance, Hitler looked to Mussolini in Italy. On May 22nd 1939, Hitler and Mussolini signed the "Pact of Steel" in Berlin which "bound each country to come to the others aid immediately." (Bullock '91) "Hitler was convinced that the effect of the treaty would be to isolate Poland by weakening the British and French resolve to come to her aid." (Bullock '91)
Now, the only country left standing without a commitment was the Soviet Union. "In the summer of 1939 the focus of European diplomatic activity became Moscow"; with the British and French competing with the Germans for Stalin's favor. (Bullock '91) "If the Soviet Union could also remain neutral in such a war between Germany and the Western Powers, this would at least enable Stalin to buy time and possibly secure territorial and strategic advantages in Eastern Europe as part of his price." (Bullock '91) The Non- Aggression Pact of 1939 was signed in August with a secret agenda between Stalin and Hitler. In the public's eye, it was a shock to see two old enemies shake hands, but the secret agenda consisted of Eastern Europe. Soviet Union would have the Baltic states of Eastern Europe as long as she remained true to the Pact whereas Germany would have Western Europe. Now a war was inevitable with Russia in a neutral corner as it seen from a public's point of view. One month later, the Germans invade Poland and Russia takes her piece: the Baltic states.
Alan Bullock. Hitler and Stalin: Parallel Lives (1991) pg. 607-620.
Helene Carrere. Stalin, Order of Terror (1981) pg. 34-53.