In the early 1980s documentary First Strike, one of the arguments made for the development of the MX missile program was that in the event of a massive Soviet attack on the land-based missile and bomber forces of the United States, the SLBM force would be "too inaccurate" to effectively target the Soviet strategic missile force. This inaccuracy would mean that the SLBM force would be restricted to targeting "soft targets" such as cities, air bases, and ports.
What was the cause of the inaccuracy of the SLBM force compared to the land-based ICBM force, and does this difference still exist today?