The contemporary European security debate has revived the classical question of how far the progressive professionalization of small armed forces converges with, or diverges from, the civilian society that sustains them. This article examines the longitudinal relationship between military professionalization and the civil-military gap across eight small states of Southeast Europe — Slovenia, Bulgaria, Romania, Croatia, Albania, Montenegro, North Macedonia and Bosnia and Herzegovina — over the period 2014–2023. Two composite, dimensionless country-year indices are proposed. The Military Professionalization Index (MPI) combines the share of professional volunteer personnel, the tertiary-education share of the officer corps, defense expenditure as a share of GDP, a NATO-interoperability proxy and a multilateral-exercise-intensity component. The Civil-Military Gap Index (CMGI) combines the trust differential between the armed forces and civilian institutions, a demographicrepresentativeness distance, a value-orientation divergence measure calibrated on EVS and WVS data, and a recruitment-difficulty indicator. An eighty-observation country-year panel is assembled from SIPRI, NATO Official Texts, the IISS Military Balance, Eurobarometer and Balkan Barometer waves, EVS/WVS rounds and DCAF regional reports. Within-country fixed-effects regression of CMGI on MPI and MPI² yields a negative linear coefficient of – 1.88 (t = –6.84) and a positive quadratic coefficient of +1.35 (t = 7.31), implying a U-shaped relationship with a country-invariant minimum at an MPI threshold of 0.697 (adjusted R² = 0.487). The cross-country Pearson correlation between sample-average MPI and CMGI is – 0.53. Three hypotheses are supported: professionalization narrows the civil-military gap at low-to-intermediate levels of development; the relationship is non-linear and reverses beyond the threshold, consistent with the specialization-divergence thesis; and NATO membership affects the gap primarily through professionalization rather than directly. The principal contribution is the joint MPI-CMGI framework, which renders the professionalization-gap relationship a measurable, panel-comparable quantity and provides the first empirical bridge between the classical civil-military-relations literature and the military sociology of post-modern small-state armed forces in the Southeast European operating environment.