Borderline as a defence line is central in countering transnational insurgency. Yet, countries in African Sub-region are lackadaisical about redefining inherited colonial borders. Primarily, the study examined the origin of Multi-National Joint Task Force (MNJTF) counterinsurgency (COIN) operational mandate as well as the linkage between border, threat of insurgency and MNJTF COIN operation in Nigeria's northeastern region. It derived data from secondary sources and method of analysis adopted was content descriptive analysis ; enemy-centric and population-centric COIN theories upheld this study. Interestingly, study reveals that in 1994, MNJTF was established by Nigeria to deal with insurgent from its northern borders and later expanded to include African neighbours. That in 2015, MNJTF was formally authorized under a new concept of operation to cover COIN with the mandate to: Create a safe environment in areas affected by Boko Haram activities; prevent expansion of insurgency activities and facilitate overall stabilization programme to fully restore state authority. However, study found four evidences linking the threat of insurgency to the porous/undefined nature of the shared borderland with Niger, Chad and Cameroon: (1) the overlapping pattern in community settlements on the borderland makes it difficult for MNJTF to effectively counter insurgents; (2) it favours illegal arms trafficking; (3) borderland forested area serves as shelter and factory base of the insurgents; (4) borderland ecological factor strengthens insurgent membership and operation in the terrain. Therefore, paper argues that with the present character of the shared borderland in northeastern Nigeria, countering insurgency would remain challenging. Paper recommends policy guide for effective COIN.
Key words: Arms trafficking, ecological factor, insurgency, counterinsurgency, Nigeria's northeast.