The decision step is made by the national command authorities based on whatever political, strategic, or operational factors they perceive and desire. The reach step is achieved by the ICBM' s ability to launch into low earth orbit and release its warhead for a ballistic supersonic reentry over the target area thousands of miles away. The hit step is completed with the successful weapon impact and detonation within the intended target area within a time frame that insures that any mobile target is still in the area. Accessing whether the target was actually destroyed is verified by satellite or aerial reconnaissance after the dust settles. To defend against such an attack requires the defender to defeat at least one of the missiles target cycle steps.
Mobility is the defensive concept behind mobile ICBMs.
For the Decide step, deterrence is the most effective and most commonly used defensive tactic. Here a defender makes sure that an attacker realizes any nuclear attack is not worth the retaliation that would rain down as result. While technically difficult, this still makes a great deal of tactical sense should deterrence fail to influence the decide step of an attacker. The only possible defense is to get out of the destructive radius of the explosion. The United States does this when it flushes the bomber force upon warning of inbound ICBMs.