In case of retaliation you want it to be delivered soon, ideally before you and the retaliation infrastructure is destroyed. If the retaliatory strike is alread

In case of retaliation you want it to be delivered soon, ideally before you and the retaliation infrastructure is destroyed. If the retaliatory strike is already in flight before the first strike can even reach the enemy territory (or sattellites, etc.) then it is much more difficult to prevent. That's why the response time is so crucial. To ensure retaliatory strike strikes fear in your enemy (pun intended) you need to convince the enemy you are capable and willing to put your ICBMs before their ICBMs can even reach you. If you have only 2 minutes you need to convince the enemy you are willing and capable to do it within 2 minutes of detecting the strike. You also want retaliatory strike to be delivered all at the same time as the best strategy seems to be in numbers. It is relatively easy to stop a single ICBM. It is completely different problem to stop hundreds of them each possibly containing multiple warheads. In this case the strategy is to deluge the enemy with warheads or, more precisely, hope they will not try to destroy you because they can never be sure to stop all out retaliatory strike.