During the development of Nike-X, controversy over the effectiveness of anti-ballistic missile systems became more prominent. Critiques of the Nike-X included an estimate that the anti-ballistic missile system could be defeated by Soviets manufacturing more ICBMs, and the cost of those additional ICBMs needed to defeat Nike-X would also cost less than what the United States would spend on implementing Nike-X. Additionally, McNamara reported that a ballistic missile system would save American lives at the cost of approximately $700 per life, compared to a shelter system that could save lives at a lower cost of approximately $40 per life. As a result of these estimations, McNamara opposed implementation of Nike-X due to the high costs associated with construction and perceived poor cost-effectiveness of the system, and instead expressed support for pursuing arms limitations agreements with the Soviets. After the Chinese government detonated their first hydrogen bomb during Test No. 6. in 1967, McNamara modified the Nike-X program into a program called Sentinel. This program's goal was to protect major U.S. cities from a limited ICBM attack, especially on one from China.