From the moment states learned to turn intercepted signals into strategic advantage, cryptography has been more than a technical craft: it has been a way of organising power. Today, more powerful computation is reshaping that contest in distinctive ways. The risk is not only that tomorrow's machines may read today's encrypted archives, but that the mere possibility of future decryption changes what rivals collect, what institutions must protect, and how long trust can reasonably last. Quantum Deterrence explains the strategic implications of post-quantum cryptography without treating it as a single technological cliff-edge. Kenan Durveil shows how encryption standards shift, why key management is often the true point of failure, and how "harvest now decrypt later" turns long-lived data into a reserve of influence. Moving across protocols, certificates, archives, and constrained communications, the book focuses on the institutional realities that determine outcomes: legacy systems that cannot be updated, vendors that set defaults for the world, and governance structures that struggle to own long-horizon risk. Written for students, general readers, and policy and security audiences, the book provides a clear framework for cryptographic migration as a coordination and credibility problem. Readers finish with a sharper understanding of which systems are most exposed, why capability gaps can produce asymmetric advantage, and what deterrence can mean when secrecy may fail after the fact. The central lesson is durable: in a world where mathematics and machines keep advancing, the decisive question is whether institutions can change as quickly as the threat model does.