The MX were a counterforce weapon, not first strike, albeit they had the ability to be if...we had 3 X more of them. The MX gave us the ability to retaliate against their arsenal...after they struck first...and gave us the option of not having to nuke their cities. And by taking out their remaining missiles...that would keep them from thinking they could win. And their existence helped us prevent their attack, and thus caused their ultimate collapse as their nuclear war winning doctrine imploded.
Today, however, the clock is being wound back to before Reagan...to our extreme danger. The Russians for example, are busy deploying an equivalent mobile missile system to the MX...the Topol-M.... while we are dismantling systems...they are deploying.... H'mmm.