Final answer:
The MK-21 reentry vehicle replaced the MK-12 in the Minuteman III ICBM system starting in 1997, as part of the strategic nuclear forces modernization.
Step-by-step explanation:
The MK-21 reentry vehicle replaced the older MK-12 reentry vehicle in the United States' Minuteman III intercontinental ballistic missile (ICBM) arsenal.
The MK-21 was introduced as part of the life extension and modernization of the Minuteman III ICBM system.
The deployment of the MK-21 began in 1997, aiming to improve the capabilities and security of the United States' strategic nuclear forces.
Hence, the Minuteman III intercontinental ballistic missile (ICBM) system underwent a strategic nuclear forces modernization in 1997, with the MK-21 reentry vehicle replacing the MK-12 in the system.