How did multiple independently targeted reentry vehicles make missile defense more difficult?

Multiple independently targeted reentry vehicles (MIRVs) significantly complicated ballistic missile defense by overwhelming defensive systems through sheer volume and tactical complexity.

By utilizing a "bus" to release multiple warheads on independent trajectories, a single missile could strike widely separated targets, forcing defenders to track and engage numerous individual reentry vehicles (RVs) simultaneously. This capability effectively expanded the "footprint" of an attack, making it difficult for a limited number of interceptors to neutralize all incoming threats.12

Furthermore, MIRVed systems were often deployed alongside penetration aids—such as decoys, chaff, and jammers—designed to confuse tracking radars. Because these defensive radars must reliably distinguish actual warheads from "confusion targets" to be effective, the presence of multiple RVs and decoys created a massive data-processing challenge. While endoatmospheric defenses can eventually identify warheads by observing how lighter decoys slow down upon hitting the atmosphere, the use of MIRVs forces defenders to manage a much higher density of objects, significantly increasing the probability that at least some warheads will penetrate the defensive shield.234