I chose North Korea for the thought experiment because of it's belligerence, not because of it's quantity of nuclear weapons.

>The Cold War being over, are you seriously proposing they need this entire arsenal in order to defend themselves against the very serious and dangerous threats of checks notes Pakistan and North Korea?

Why did you choose to ignore the Chinese who are currently rapidly expanding their nuclear arsenal. The world is on the precipice of a new cold war between Beijing and Washington.

>If so, then how come the US still reserves the right to a pre-emptive nuclear strike? How come it has not adopted a "no first use" policy like China and India?

A "no first use" policy is worth less than the paper it would be printed on. No one serious actually believes in them.