Complex sentence: Elf, at that time owned by the French state, then undertook a policy to rebuild and promote French motor racing and so began a new long-term relationship with Renault, which was also owned by the French government at the time.
Split into simpler sentences: Elf, at that time owned by France, then undertook a policy to rebuild and promote French motor racing. This began a long-term relationship with Renault, which was also owned by the French government at the time.