[philipkglass]

A country with 5,000 nuclear weapons could conduct a first strike against a country with 1,500 nuclear weapons, on a 2:1 ratio, and still have 2,000 nukes in reserve for further strikes. This is why the nuclear arms race happened between the US and the Soviet Union in the first place. Any disparity in the deployed arsenals gives the side with more the advantage. So if China ever decides to expand beyond 1,500, the strategically sound move for the US would be to start building more, to match the Chinese production. It would be tragic, but it's not impossible.

Submarine launched ballistic missiles and mobile missiles on land (train or truck based) break this race. If you don't know where all the enemy launchers are, having enough weapons to hit them all in a first strike doesn't matter much. That's why the US has a strong deterrent even though Russia has more warheads than it does [1]. The mobile weapons can't be guaranteed destroyed and a retaliatory strike from them will still be devastating. The US's mobile deterrent is based on submarine launched ballistic missiles but it has designed mobile land based weapons in the past, and other countries (e.g. Russia) still have mobile land based weapons.

[1] https://en.wikipedia.org/wiki/Russia_and_weapons_of_mass_des...

[PeterisP]

It does mean that a change in intelligence (whether technical or human) that reveals the location of enemy launchers can suddenly enable this first strike - for example, if someone develops some technology capable of sensing where the US missile submarines are located, then you can't rapidly increase the size of nuclear arsenal fix the issue, you have to have the reserves pre-built.