No ABM system has demonstrated success against either MIRVs or deployed PENAIDS (much less 53T6), but more to the point BM's of any sort are not a first strike weapon (given US and RF radar nets, 24/7 US launch detection capability, and part-of-the-day RF launch detection capability).

If the US has a first strike weapon, it's a fleet of nuclear-capable stealth bombers employing GPS-guided laydown-capable munitions, coupled with a pretty nasty set of eyes in the sky to find mobile launchers. F-35 is also stealth and MAY get a B61 interface to replace NATO Tornados and F-16s, etc.