Covert cross border infiltration plays a critical role in modern warfare. The methods counter U.S. military technical and firepower advantages by hiding behind the international understanding about the sanctity of borders. Cross-border infiltration enables enemies to seize and maintain the offensive initiative from bases in adjacent safe haven countries. Covert cross-border infiltration allows states such as Russia to pursue aggressive geopolitical policies while maintaining plausible deniability to the international community. For non-state actors, cross border infiltration provides a survivable and practical way to achieve their political goals.This study examined how U.S. Special Forces and First Field Forces conducted border denial operations in the Central Highland "tri-border" region during the Vietnam War. Despite the development of cross-border operations doctrine during the Vietnam War, this study concludes the U.S. Army discarded and discounted their lessons learned. The U.S. Army today may achieve strategic border denial by arranging tactical actions causing the adversary to suffer the cost of infiltration without gaining any benefits. Using the deep-close-support operational framework, interdiction, barrier emplacement, and a learning border security system, the U.S. Army can disrupt enemy safe havens, neutralize infiltrators, and build capable host nation border security forces.Contents: 1. Introduction 2. Literature Review 3. Analysis 4. Conclusions and RecommendationsDenying the enemy an unmolested journey from state to state is essential. This denial increases security by allowing domestic conflicts to be resolved without outside forces aggravating the situation. Counterinsurgency operations are futile if fighters and their weapons pour into the contested environment from neighboring safe haven countries. Therefore, the U.S. military needs to be prepared and proficient in countering cross border infiltration. America's previous efforts to seal the Vietnamese border from communist incursions may prove instructive.Successful border denial operations require sufficient doctrine, practice, and learning. Sufficient doctrine provides a set of fundamental principles to guide operations enhancing operational effectiveness. It is collection of best practices learned from experience and addresses the complexity of the contemporary operating environment. Successful border denial operations involves practice and learning. U.S. military forces must expect adversaries to adapt to U.S. counter infiltration methods. Relying on experience is not enough. Denying the border to infiltrators necessitates that organizations not only embrace learning, but also learn how to learn faster. I Field Forces border operations in the Central Highlands during the Vietnam War exemplified this model.