Complex sentence: All male Bulgarian subjects were eligible to serve in the army when they reached the age of 20 when they were conscripted for a period of two years in the infantry and three years in other branches of the Active (Standing) Army.
Split into simpler sentences: All male Bulgarian subjects were eligible to serve in the army when they reached the age of 20. At that age, they were conscripted for a period of two years in the infantry and three years in other branches of the Active (Standing) Army.