‘Who is the enemy?’ This is the question most asked in modern warfare; gone are the set-piece conventional battles of the past. Once seen as secondary to more traditional conflicts, irregular warfare (as modified and refashioned since the 1990s) now presents a major challenge to the state and the bureaucratic institutions which have dominated the twentieth century, and to the politicians and civil servants who formulate policy. Twenty-first-century conflict is dominated by counterinsurgency operations, where the enemy is almost indistinguishable from innocent civilians. Battles are gunfights in jungles, deserts and streets; winning ‘hearts and minds’ is as important as holding territory. From struggles in South Africa, the Philippines and Ireland to operations in Iraq, Afghanistan and Chechnya, this book covers the strategy and doctrine of counterinsurgency, and the factors which ensure whether such operations are successful or not. Recent ignorance of central principles and the emergence of social media, which has shifted the odds in favour of the insurgent, have too often resulted in failure, leaving governments and their security forces embedded in a hostile population, immersed in costly and dangerous nation-building.