Complex sentence: The Soviets' first important assignment for Navachine sent him to Spain in 1921; as a diplomat, Navachine negotiated with Spain to exchange its petroleum reserves with the Soviet Union rather than Royal Dutch Shell.
Split into simpler sentences: The Soviets' first important assignment for Navachine sent him to Spain in 1921. There, as a diplomat, Navachine negotiated with Spain to exchange its petroleum reserves with the Soviet Union rather than Royal Dutch Shell.