Your question exposes a few misunderstandings about WW2. Hitler was in the business of poking the hornets nest. Remilitarising the Rheinland, blatantly breaking the conditions in the Treater of Versailles, the Anschluss of Austria, demanding the Sudetenland from Czechoslovakia. The Allies didn't move in any significant way to prevent any of this.

In fact, you can argue France and Britain declared war on Germany on a technicality, but it's neither here nor there, really. They declared war on Germany after Hitler invaded Poland in 1939. Britain and France were on the record as this being a line in the sand: If Poland was invaded, they'd join the war.

There were many reasons for the invasion of Poland, but to directly address your question: Germany and the Soviet Union didn't share a border. To invade the USSR, Hitler had to go through Poland. But there were several reasons for the invasion of Poland.

In addition, only a quarter century earlier, Poland didn't exist, and the land that later became Poland was part of the German and Russian Empires. If you look at a map from that period, you can actually see that there's a piece of Germany inside Poland on the coast of the Baltic Sea. That's because large amounts of west Poland used to be a part of the Kingdom of Prussia (not Russia), a major precursor state to the modern German state in the 1800s. A main focus was the reclamation of the then Polish city of Gdansk, known to the Germans as Danzig, which was right between the German exclave of Ostpreussen and Germany proper. Germany had actually suggested extending the Reichsautobahn through Gdansk but the Polish government rejected it.

Another important thing to keep in mind is that the Nazi rhetoric about the evils of Communism, Bolshevikism, and Soviet Russia was largely a matter of political convenience. While it's true that a lot of Nazi ideology was in direct conflict with Soviet ideology, and a large part of Nazi ideology was based on promoting the inevitable defeat of the Soviet Union and the supremacy of the German people, that didn't stop the Nazis signing a non-aggression pact with the Soviet government and dividing Poland with them.

Back to your question though: After France and Britain declared war on Germany in response to the invasion of Poland, conflict didn't break out right away. Germany was busy in the east and the Allies were busy dealing with the fact they were now in a war they didn't really want to be in. The so-called "Phony War" continued until May 1940 when Germany successfully invaded France and Britain and her Empire were on their own.

To sum up: The war between the Allies and Germany was a byproduct of Hitler's ambitions in the East, and the sincerity of the German leadership to actually pursue war with the Soviets wouldn't bear fruit until the invasion of the USSR AKA Operation Barbarossa in 1941.