What was seen as a particular threat to the balance of terror during the salt talks?

During the Strategic Arms Limitation Talks (SALT) in the 1970s, the deployment of multiple independently targetable reentry vehicles (MIRVs) by the United States was seen as a particular threat to the balance of terror. MIRVs allowed a single missile to carry multiple warheads, increasing the potential for a first-strike capability and undermining the principle of mutually assured destruction (MAD). This development raised concerns in the Soviet Union about the stability of the nuclear deterrent and prompted fears that it could lead to an arms race, jeopardizing the progress made in arms control negotiations.