While no state has yet to strike an enemy with its nuclear arsenal after 1945, political science still fails to adequately explain the significant and peculiar variation in advocacy among leaders for using nuclear weapons, particularly in settings where nuclear retaliation is not plausible. Prevailing literature has claimed that outside of nuclear deterrence, leaders have been restrained by either a normative taboo proscribing nuclear weapons-use (NWU), or otherwise by the conviction that long-term consequences would be unacceptable. This dissertation argues instead that leaders' advocacy levels for NWU are chiefly determined by strategic decisiveness - the extent to which leaders believe the escalation could decisively resolve a major security problem in a way existing conventional alternatives could not. Decisive NWU would be expected to promote the state's relative power, demonstrating its competence, capabilities, and resolve in defeating the enemy and stewarding the international order. Conversely, indecisive NWU would be expected to signal ineptitude, with immediate consequences, undermining the state's deterrent credibility and unraveling its coalitional security arrangements. This dissertation process-traces decision-making through three case studies derived from archival work to show that both civilian and military leaders struggle to find an effective and efficient use for their nuclear arsenal, relative to existing conventional alternatives, and that such disappointing lessons are repeatedly incorporated into the state's nuclear doctrine to become assumptions for future decision-making.