However, the most significant issue is that nuclear deterrence simply doesn’t work the same way in the context of a major power-weak state dyad. Nuclear deterrence was effective with Russia and China because both saw themselves as massive and great civilizations in which nuclear weapons were guarantors of ultimate security, not instruments of first response. The risk of uncontrolled escalation created a disincentive against threatening the core interests of rival nuclear powers, and reduced (but hardly eliminated) the threat of major power war.
The incentive for North Korea is exactly opposite. Far from avoiding threatening America’s core interests, doing so directly advances Pyongyang’s own strategic goals. This is because the costs to the United States of intervening will greatly outweigh those of acquiescing to what are, relative to major power competitors, modest North Korean objectives (even though the long-term consequences for the United States’ position in Asia is profound). Moreover, unlike with America’s major power rivals, any level of American military intervention taken against North Korea would necessarily be interpreted by Pyongyang as an existential threat to regime survival, meaning that dramatic escalation is assured and not merely a risk. In short, North Korea will increasingly engage in hostile aggression below the nuclear threshold, without fear of conflict. The bottom line is that the United States will be deterred, not North Korea, despite the wide gap between their respective nuclear capabilities.
A better example than China or Russia would be India and Pakistan, were Pakistan only aggressive toward distant Indian allies and not India itself.