Complex sentence: Many of those killed were leaders of the SA ("Sturmabteilung"), the Nazis'own paramilitary Brownshirts organization; the best-known victim was Ernst Röhm, the SA's leader and one of Hitler's longtime supporters and allies.
Split into simpler sentences: Many of those killed were leaders of the "Sturmabteilung" ("SA "), the Nazis'own paramilitary organization, colloquially known as the"Brownshirts" from the color of their uniforms. The best-known victim of the purge was Ernst Röhm, the SA's leader and one of Hitler's longtime supporters and allies.