This pact between hitler and stalin was very frightening to other Allied powers because two very powerful nations are joining together to do who knows what to other smaller european nations. Then, when the USSR and Germany invaded Poland from both sides, it was clear that they didn't sign the pact for defence and protection, but to invade other smaller country. But it wasn't later revealed that Hitler only did this for invading other smaller countries. Hitler did this so that he wasn't going to be attacked by both sides of Europe and later, broke the pact by invading the USSR. The point is, this is one of the reasons WW2 started (there are many more reasons too)
