Simplify the following text for a Grade 9 native English-speaking person.
Original: Under the leadership of Heinrich Himmler between 1929 and 1945 , the SS grew from a small paramilitary formation to one of the largest and most powerful organizations in the Third Reich .
Simplified: Heinrich Himmler was its leader from 1929 to 1945 . During that time , the SS grew from a small paramilitary formation to become one of the largest and most powerful organizations in Nazi Germany .