How can you explain Hitler’s success in the early days of World War II?

At the initial stage of the war, Hitler was able to take advantage of the surprise effect. In addition, the best generals were under his leadership. The Soviet command at that time did not have experienced commanders, which was associated with the repressions of 37-39, when a significant part of the experienced officer corps was arrested. These factors, to one degree or another, were reflected in the strategic situation at the initial stages of the war.