Original sentence: This was cancelled because of delays in achieving a reliable circumlunar flight (only the later Zond 7 and Zond 8 members of the programme were successful) and the Apollo 8 mission had already achieved that step in the Space Race
Edited sentence: This was cancelled because of delays in achieving a reliable circumlunar flight (only the later Zond 7 and Zond 8 members of the programme were successful) and the Apollo 8 mission had already achieved that step in the Space Race with the United States gaining that prestige first