From what I remember of the difficulties of the Star Wars program - ICBMs are most vulnerable and easiest to spot during their boost phase - which tends to happ

From what I remember of the difficulties of the Star Wars program - ICBMs are most vulnerable and easiest to spot during their boost phase - which tends to happen, rather inconveniently, over your enemies territory. The RVs themselves are small, hardened and move incredibly quickly and are also surrounded by a plasma shield during reentry which may interfere with targetting with a directed energy weapon.