So, when GER and the USSR invaded Poland after Molotov-Ribbentrop, why did GER become the main target? why did they not also declare war on the USSR? I believe there may have been a non-agression treaty between FR and the USSR but,I'm not sure.
Short answer: because Britain and France saw Germany as a threat and not the USSR.
I'm basing this answer off Peter Jackson's (a historian, not the hobbit!) book which details the French response to Nazism. Essentially they were convinc Led that Germany would eventually invade France. They made some assessments and worked out that if this were to happen France could just survive. However, unlimited German access to oil would tip the scale in Germany's favour. The French and British didn't want Germany in Poland because they thought that this would be the beginning of a war which concluded with the Germans in Romania where they could seize oil. France and Britain viewed this as a worst-case scenario. While the USSR was not on the greatest terms with Britain and France, at least they had no recent history of major hostilities. Furthermore, the USSR did not border France and finally, it was not allied to Italy who was assumed could steal France's African empire in a heart-beat.
Also, Britain and France were still injured by the Depression, and as evident by their 1930s foreign policy of appeasement, were fearful to engage with one army let alone two.
I know this has been asked before but I couldn't find it in the FAQ. Luckily for you I was able to find a few of the previous questions in the search somehow. (seriously I can't believe the search function actually worked). Hopefully you will get some more answers as well.
Why did England and France only declare war on Germany while Russia invaded Poland too ?