Global food production relies heavily on fossil-energy-derived nitrogen (N) fertiliser, creating a structural vulnerability to energy and geopolitical shocks. Disruptions rapidly raise fertiliser costs, threaten food production, and disproportionately affect smallholders and import-dependent countries. This paper argues that nitrogen is no longer only an agronomic input or an emissions problem, but a strategic vulnerability at the intersection of food, energy, and soil security. We review what is established in nitrogen science, where uncertainties remain, and why technical knowledge has not translated into enduring improvements in nitrogen productivity. We then propose a leverage-based research and policy agenda: buffering current systems against shocks, improving nitrogen flows within existing production systems, changing feedbacks and incentives, restructuring nutrient and energy pathways, and ultimately redefining agricultural goals around sustained production under lower fossil nitrogen dependence. The central task is not simply to maximise nitrogen-use efficiency, but to build food systems that maintain yield and profitability, reduce exposure to fossil-energy and fertiliser-market shocks, and protect the soil nutrient capital on which long-term productivity depends. Reframing N this way shifts the agenda from isolated technical fixes to coordinated changes in agronomy, breeding, soil biogeochemistry, infrastructure, markets, and policy.