Complex sentence: The Soviet nuclear program would have eventually been able to develop a nuclear weapon without the aid of espionage, but a basic understanding of the usefulness of an atomic weapon, the sheer resources, and the talent did not develop until much later.
Split into simpler sentences: The Soviet nuclear program would have eventually been able to develop a nuclear weapon without the aid of espionage. But a basic understanding of the usefulness of an atomic weapon, the sheer resources, and the talent did not develop until much later.