This United States Marine Corps (USMC) manual, Marine Corps Tactical Publication MCTP 3-03D Security Cooperation August 2020, is designed to assist Marines at every level in the execution of their duties to prioritize, plan, execute, and assess security cooperation engagements with partner nations in support of U.S. national security objectives. This publication underscores the importance of security cooperation as a way to help set the theater to enable leaders and commanders to expand the range of available operational options by enabling access and developing the capabilities and capacities of partners and allies globally. In addition, the publication emphasizes the unity of effort and action needed for success in security cooperation that require coordination across Marine Corps, joint, and interagency organizations that are not part of Marines’ formal chain of command. The intent of the publication is to outline security cooperation guidance and lexicon, as well as Marine service component planning considerations, such as legal and political considerations, tools and resources, and the Marine Corps’ security cooperation planning and execution process. While the techniques and procedures captured within this publication reflect current best practices, it is neither intended to be all-encompassing, nor prescriptive. To support campaign plan objectives; security cooperation activities can include and address a wide range of challenges across the range of military operations, from direct military confrontation, peace keeping operations, and building partner capacity and capability, to providing humanitarian assistance and disaster relief, and stability operations. Security cooperation activities can set the conditions for success in contingencies and preparing for catastrophic events. Many generations of Marines have engaged with international partners, in peacetime and war, to build interoperability and to enhance the capacity and capabilities of our partners. The Marine Corps will continue to conduct security cooperation activities across the range of military operations with Marine air-ground task forces or other task-organized force packages enabled for security cooperation. Marine Corps security cooperation activities are planned, executed, and assessed at all levels—from the regional MARFOR and Service headquarters, to Marines task-organized and equipped to conduct a mobile training team or advisor mission with a partner. As such, this publication applies to Marines and civilians across the total force. This publication cancels Marine Corps Interim Publication 3-03Di, Security Cooperation, dated 21 July 2015 and its associated change page and erratum.