How did multiple independently targetable reentry vehicles make arms control more difficult?

Multiple independently targetable reentry vehicles (MIRVs) significantly complicated arms control by enabling a single ballistic missile to carry several nuclear warheads, each capable of being directed toward a separate target. This technological advancement undermined strategic stability and made arms limitation negotiations more difficult for several reasons:12

• Swamping Defenses: MIRVs could overwhelm antiballistic missile (ABM) systems, rendering defensive measures ineffective and forcing a shift in focus toward offensive capabilities.32
• Verification Challenges: Because MIRVs allowed for a massive increase in the number of warheads without necessarily increasing the number of missile launchers, it became harder for negotiators to accurately compare the destructive power of opposing nuclear arsenals.42
• Strategic Vulnerability: The deployment of MIRVs created concerns that land-based ICBMs could become vulnerable to a preemptive strike, as one side could potentially destroy multiple enemy silos with a single, highly accurate MIRVed missile.43
• Technological Upgrades: Even when treaties limited the number of missile launchers, they often failed to restrict the "MIRVing" of those launchers, allowing the arms race to continue through technological upgrades rather than just the production of new missiles.2