1. Russia does not sign onto the August 23rd 1939 non aggression pact through some sort of nuance point. (e.g. arguments over Romanian, Baltic territory and/or the drawing of the line of Poland). Hitler nonetheless goes and invades Poland, and subsequently the SU retaliates assisting the Poles.
This probably wouldn't prevent the Western Allies from declaring war themselves, but they may be a bit reluctant.
2. Hitler holds invading into Poland in September 1939 (maybe they the Nazi's are given Danzig, maybe he has a change of thought). Some sort of event occurs on the Soviet/Polish border, perhaps an ultimatum by Stalin and in this instance it is the Germans who will end up assisting the Poles against Russia. I honestly don't think this has much merit although Stalin's army was indeed an offensive army, it wouldn't have been prepared (which is why they did sign the pact with Germany IRL).
3. The Tripartite Pact is signed earlier, perhaps along with the Anti con-intern pact, and then Japan has those border clashes with the SU. Again, imo, not that believable (as Germany would have lost the element of surprise and the ability to even strike - they would have to go through Poland.)
4. The other way and IMO the most likely way this could have materialized is if the British decided to sue for peace after Dunkirk pocket was completely destroyed, or if Nazi Germany gave them an acceptable treaty (the latter of which I believe would have been extremely unlikely). As for the destruction of the Dunkirk pocket, that is a what if question for the ages. But one thing was for sure, in the minutes in the British parliament, there was consideration given to surrender.