Complex sentence: After the war, Oppenheimer became chairman of the influential General Advisory Committee of the newly created United States Atomic Energy Commission, and used that position to lobby for international control of nuclear power to avert nuclear proliferation and a nuclear arms race with the Soviet Union.
Split into simpler sentences: After the war, Oppenheimer became chairman of the influential General Advisory Committee of the newly created United States Atomic Energy Commission. He used that position to lobby for international control of nuclear power to avert nuclear proliferation and a nuclear arms race with the Soviet Union.