Since the Cold War, nuclear powers have said "don't mess with me" using the Nuclear triad, nuclear warheads on bomber planers, missile silos, and in submarines. Of these, the most important is submarines. Silos and planes could be wiped out in a first-strike attack, but submarines are stealthy.
If the setting is earth of the not-too-distant-future, what would be the most plausible way for a superpower to strike first and prevent their opponent's second strike?
My ideas so far divide into three categories: 1) sabotaging the subs, 2) blocking the missiles, 3) doxxing the subs.
Sabotaging the subs is probably most realistic. The adversary gets a spy/mole inside the nuclear submarine program, and does something to some vital system (which?) to scuttle all the subs simultanæously.
Blocking the missiles is what most budget goes into researching, but no country can currently block a high percentage of incoming missiles. Hundreds of billions have been spent on this, and it's gotten nowhere, so "one day, someone invented a perfect missile defense" feels like poor worldbuilding.
Unveiling the subs using some sort of enhanced Magnetic Anomaly Detector probably isn't that realistic.