What is the primary mission of fleet ballistic missile submarines?

The primary mission of fleet ballistic missile submarines is to deter war by maintaining a credible and survivable second-strike capability. This role helps to ensure that potential adversaries recognize the severe consequences of launching a nuclear attack against the United States or its allies. By remaining undetected and able to launch ballistic missiles from underwater, these submarines create a powerful deterrent effect, effectively contributing to global stability and preventing conflict.

While the other choices address important aspects of military strategy—such as engaging enemy submarines or providing backup for air commands—they do not capture the essential purpose of fleet ballistic missile submarines, which is fundamentally about deterrence and ensuring peace through the threat of significant retaliation. The emphasis on a first strike capability also diverges from their actual mission, as these submarines are designed more for assured retaliation in response to an attack rather than initiating conflict.