The electromagnetic spectrum has become a cornerstone of modern technological society, underpinning critical functions in both national defense and civilian infrastructure sectors. It is, therefore, a strategic imperative to have dominance and security in this domain Computational Electromagnetics (CEM), an engineering art in which one is able to solve Maxwell's equations numerically for simulating complex electromagnetic phenomena, has proven to be an enabling technology that is essential for these endeavors. In this paper, we describe the centrality of CEM in reaching the goals of ensuring national defense and critical infrastructure against the ever-expanding threat milieu. This paper recombines recent advances and identifies critical thematic domains where CEM is pushing transformative progress. Such developments also handle design and analysis of low observable platforms and counter stealth technologies, fusion of CEM with artificial intelligence (AI) enabling cognitive electronic warfare and spectrum dominance, modelling and mitigation of threats like high altitude electromagnetic pulse (HEMP) and intentional electromagnetic interference (IEMI) for infrastructure protection, as well as novel state of the art antenna systems for superior communication, sensing and navigation. Through the review of the key applications, this paper shows the development of CEM applications, from a special-purpose design tool to a strategic asset for virtual prototyping, threat evaluation, and mission planning. In addition, this article pinpoints the future emerging research directions, such as real-time multiphysics simulation, AI-accelerated solver, and the inception of quantum-enhanced electromagnetics. In the end, this work surmises that innovation in CEM remains essential to asserting a continued decisive advantage in the CEM domain and combating the erosion of modern society’s infrastructure hardening over the years.