Complex sentence: In 1915 he graduated from Peterhof Military School, and participated in World War I. In July 1917 Parts joined Estonian national units, during the German occupation in 1918 he organized the underground Defence League.
Split into simpler sentences: In 1915 he graduated from Peterhof Military School, and participated in World War I. In July 1917 Parts joined the Estonian national units. During the German occupation in 1918 he organized the underground Estonian Defence League.