Why an ICBM is a dangerous anti-ship weapon

Would intercontinental ballistic missiles make a heavily protected modern battleship or carrier group irrelevant?

They are powerful, but using them conventionally at sea creates a severe risk that the launch will be interpreted as the start of a nuclear strike.

The problem is one of warning and escalation rather than simple destructive power. A state observing one or several ICBM launches cannot immediately know whether the warheads are conventional, nuclear, aimed at a fleet, or opening a wider strategic attack. Saturating a carrier group's defences would make the launch pattern still more alarming. ICBMs can therefore impose costs and deter, but using them tactically risks forcing an opponent to make a nuclear decision under severe uncertainty.