Complex sentence: The first wave of German attackers counted only about 10,000 men, but Germany's luck in achieving complete surprise, and the lack of preparedness of Norway for a large-scale invasion of this kind, gave the German forces their initial success.
Split into simpler sentences: The first wave of German attackers counted only about 10,000 men. Due to the surprise, and the lack of preparedness of Norway for a large-scale invasion of this kind, gave the German forces their initial success.