Complex sentence: Use of the Totenkopf as a military insignia began under Frederick the Great, who formed a regiment of Hussar cavalry in the Prussian army commanded by Colonel von Ruesch, the Husaren-Regiment Nr.5 (von Ruesch).
Split into simpler sentences: Use of the symbol as a military insignia began with the cavalry of the Prussian army under Frederick the Great. Frederick formed a Hussar regiment commanded by Colonel von Ruesch, the Husaren-Regiment Nr.5 (von Ruesch).