This study examined the impact of non-state actors (NSAs), particularly terrorist groups, on international security. Given the changing shifts in global politics, non-state actors have become increasingly influential, disrupting conventional security frameworks. This paper explored the rise of NSAs, their strategies, tactics, and their overall impact on international security. The study adopted constructivism as its theoretical lens. This perspective challenges the deterministic views of anarchy presented by realists, suggesting that anarchy's meaning and implications are socially constructed. Constructivism sheds light on how terrorist groups construct their identities, ideologies and norms; how these identities influence their actions, and how states perceive and respond to these threats. The study used a descriptive research design, with secondary data sources, and a content analysis method. The findings of this study revealed a many-sided evolution in international security. Non-state actors (NSAs) have emerged as powerful forces, exploiting state weaknesses and societal divides to embed themselves within communities and promote extremist ideologies. The findings of the study also show how NSAs use digital platforms to spread propaganda, coordinate activities, and disrupt critical government infrastructure. Findings also review that traditional warfare paradigms have been significantly disrupted, with NSAs engaging in urban warfare and launching indiscriminate attacks on civilian populations. The study recommended the need for international collaboration, strengthened state governance, and proactive measures to effectively counter the evolving threats posed by non-state actors (terrorists) to safeguard global peace and security.