This field manual establishes doctrine (fundamental principles) for tactical counterinsurgency (COIN) operations at the company, battalion, and brigade level. It is based on lessons learned from historic counterinsurgencies and current operations. This manual continues the efforts of FM 3-24, Counterinsurgency, in combining the historic approaches to COIN with the realities of today's operational environment (OE)-an environment modified by a population explosion, urbanization, globalization, technology, the spread of religious fundamentalism, resource demand, climate change and natural disasters, and proliferation of weapons of mass destruction. This manual is generic in its geographic focus and should be used with other doctrinal sources.* Chapter 1, Operational Environment of Counterinsurgency, defines insurgent and counterinsurgent while using the operational variables and mission variables to describe the OE. Finally, it stresses developing a culture capability for Soldiers and leaders.* Chapter 2, Foundations of Insurgency, categorizes insurgent groups by their components-elements, dynamics, and strategies and their manifestations-tactics, strengths, and vulnerabilities.* Chapter 3, Foundations of Counterinsurgency, covers the seven lines of effort, tactical considerations, clear-hold-build operations, and counterinsurgency phases.* Chapter 4, Planning in Counterinsurgency, arguably the most important chapter, covers planning for tactical units during counterinsurgency operations. It also covers planning horizons and targeting.* Chapter 5, Offensive Operations in Counterinsurgency, addresses offensive techniques used by tactical units during counterinsurgency operations.* Chapter 6, Defensive Operations in Counterinsurgency, addresses defensive techniques used by tactical units during counterinsurgency operations.* Chapter 7, Stability Operations in Counterinsurgency, addresses stability techniques used by tactical units during counterinsurgency operations.* Chapter 8, Working with Host Nation Security Forces, covers the seven steps (MORTEAM) units use to train, advise, and partner with Host Nation security forces.The target audience is commanders, staff, and Soldiers of US Army units up to brigade level.This manual applies to the Active Army, the Army National Guard (ARNG)/Army National Guard of the United States (ARNGUS), and the US Army Reserve (USAR) unless otherwise stated.