History questions

I was a history major many decades ago, but I changed my mind. Basically, I didn't want to end up doing research next to rats at certain archives. However, I've often had questions about certain historical events that puzzled me. A good example would be why the Normandy invasion came so late into WWII? Also, was it really so urgent? We all know the Russians had already severely weakened the NAZIS a couple of years earlier at the Battle of Stalingrad. My thought has been that perhaps we went in to stop Soviet expansion more than NAZI, but that's just me thinking. I will stop here and revisit sometime in the future.