Tehran Conference promised Stalin that a western front would open by May 1, 1944. The D Day Invasion was over a month later than promised. The Russians already won the Battle of Stalingrad in the depths of the previous winter. This was the turning point in WWII for the Nazis. From that day forwards, they were doomed. Russia created the Katyusha rockets right on the eve of the German invasion. This weapon system heralded the Russian advantage in the post war rocket race which Russia proved superior for many years.