First-strike weapons destroy your enemy's ability to retaliate. Second-strike weapons preserve your ability to retaliate after a first strike.
Missile defense systems constitute first-strike technology. Suppose you build an effective missile defense system. You could launch a pre-emptive nuclear strike against your enemies. If your enemies are growing stronger relative to you then you are incentivized to launch a nuclear strike against them, before they can build their own missile defense system.
It is impossible to defend a missile silo against a nuclear first strike because the position of a ground-based missile silo is fixed and few things provide adequate armor against a nuclear warhead. Airfields for bombers are even harder to hide. Submarines are relatively difficult to locate and destroy. Submarines are too small to launch a massive, overwhelming first strike. Submarines are second-strike technology.
First-strike weapons promote instability because they incentivize actors to strike first. Second-strike weapons promote stability because they make it safer for actors to wait. The more you can wait the less likely you'll be to gamble everything on a desperate first strike or to retaliate against false alarms.
This doesn't mean we should put all of our doomesday devices on submarines. That's a single point of failure. A diverse arsenal of delivery systems is more robust than relying on a single technology. China's submarines are (for now) less advanced than Russia's and the United States'. If China put its nukes on submarines it is plausible that its enemies could wipe them all out in a pre-emptive strike. This may be related to why China is currently expanding its ground-based nuclear missile capacity.
A world of second-strike technologies tends to incentivize peace. A world of first-strike technologies tends to incentivize pre-emptive strikes.
Perfectly effective missile defense is implausible against a peer adversary (excepting a successful pre-emptive cyberattack), but we can imagine a world where the United States deploys a successful missile defense system against a non-peer adversary against North Korea. ↩︎