Complex sentence: By 1959, the nuclear stockpile held 12,305 weapons, of which 3,968 were in AEC custody and the remaining 8,337 were held by the Department of Defense, and the total yield of the stockpile was in excess of 19,000 megatons.
Split into simpler sentences: By 1959, the nuclear stockpile held 12,305 weapons, of which 3,968 were in AEC custody and the remaining 8,337 were held by the Department of Defense. The total yield of the stockpile was in excess of 19,000 megatons.