This study examined trans-border arms smuggling across the Seme–Idiroko corridors and its implications for Nigeria’s national security from 2015 to 2025. Anchored on Ernst Haas’s Neo-Functionalist Theory and Complex Adaptive Systems (CAS) model, the research utilized a descriptive, convergent parallel mixed-methods design integrating survey and historical approaches. The target population (N = 34,001) comprised frontline security personnel (Customs, NDLEA, Police) and residents across Seme, Idiroko, Krake, and Igolo. Using proportional stratified random sampling, 400 questionnaires were administered, yielding 393 validated instruments (N = 393), complemented by 14 Key Informant Interviews (n = 14). Quantitative data were analyzed using descriptive statistics, while qualitative field narratives underwent narrative-content analysis. Findings revealed that continuous small arms trafficking severely degrades border security infrastructure (X = 4.21, SD = 1.05) as syndicates forcefully clear secondary bypasses. A direct causal link was established between Southwestern border vulnerabilities and internal security decline in Lagos and Ogun States (X = 4.18, {SD} = 1.06), as smuggled weapons fuel urban cultism, kidnapping, and armed robbery. Furthermore, current countermeasures were deemed largely ineffective (X = 4.22, SD = 1.01) due to severe technological deficits (X = 2.17), manual inspection dependencies, and inter-agency rivalries. Enforcement is severely constrained by extreme geographic porosity (X = 4.45) across thousands of unmapped bush paths and institutional compromises (X = 4.39). The study concludes that unilateral, top-down policies are ineffective against adaptive criminal networks. It recommends phasing out manual patrols in favour of drone surveillance and non-intrusive cargo scanners, creating a centralized inter-agency intelligence database, and institutionalizing bilateral security cooperation with the Republic of Benin.