The nature of war remains timelessly unchanged. However, its form is constantly evolving. We are now moving from conventional to hybrid warfare, where there are no boundaries and limitations. Now, theoretical and practical understanding is needed for deterrence (strategic dimension) and defense (operational/tactical). The lessons learned so far from the war in Ukraine are thoroughly analyzed. The Greek choices between deterrence and defense after 1974 give the author the opportunity to propose a new military philosophy for the present and the next decade. The series SMALL INTRODUCTIONS includes reference books, written by experts, which provide reliable and in-depth information on topics of immediate interest. Simple but not simplistic, scientific but not inaccessible, concise but also comprehensive, the series offers the modern reader the keys to deeply understand our complex and changing world.