Why are some nuclear weapon states attacked more than others? Reviewing attacks against them shows that nuclear-armed democracies are attacked more than non-democratic ones, indicating that state regime type may play a role in nuclear deterrence credibility. This is largely ignored in the nuclear deterrence literature despite many conceptual similarities to conventional armed conflict, which has numerous theories on regime type influencing state conflict behaviour. In this dissertation, I seek to explain the disparity in attacks on nuclear weapon states with a novel theory that state political accountability mechanisms influence nuclear deterrence threat credibility. I hypothesize that democratic states are more sensitive to the costs of fulfilling their nuclear deterrence threats in low-intensity conflict scenarios due to popular accountability. Non-democratic nuclear weapon states can make more credible threats due to being accountable to an elite audience instead of the general population, allowing them to engage in cost-insensitive behaviour. To accomplish this, I use process tracing within a broader comparative case study framework to study crises affecting four nuclear weapon states during the 1945-2018 period. For each of these four states (Israel, India, Pakistan, and North Korea), I study the makeup of their government, including their domestic audiences, their nuclear deterrence threats, and command and control structures. I then compare pre- and post-nuclear deterrent acquisition crises targeting them and evaluate the rationales of decision-makers for both the defending and challenging states, particularly how the latter perceived the former's cost-sensitivity. The results indicate that, as anticipated, nuclear-armed democracies face greater difficulties in having their nuclear deterrence threats considered credible in low-intensity crises than their non-democratic counterparts. However, each case comes with notable caveats. Israel's nuclear opacity policy obscures whether government restraint is due to accountability or potential international reactions, while the Indian government often goes against popular sentiment in demonstrating restraint. While Pakistan and North Korea have both successfully deterred even low-intensity attacks against them due to the perception that they are sufficiently cost-insensitive to use nuclear weapons against any attack, the latter's aggression has led to it being considered sufficiently dangerous as to encourage potential pre-emptive strikes against it.