IntroductionSince the Russian Federation invaded Crimea in March 2014, analysis and commentary on the concept of warfare have increased exponentially.1 An Internet search will identify hundreds of entries covering the phenomenon. Hybrid warfare has become the most common term used to try and capture the complexity of twenty-first-century warfare, which involves a multiplicity of actors and blurs the traditional distinctions between different types of armed and even between war and peace. Hybrid warfare has ceased to be a topic only for military strategists, as it has now entered the broader public domain and become a major security concern for Western governments. Both NATO and the European Union (EU) are working on strategies to strengthen defensive capabilities and prevent attacks.This article seeks to clarify the different ways in which the term warfare and related terms have been used by scholars and policy analysts and summarize discussion on the topic to date. The paper will examine, in particular, the Russian approach to warfare as demonstrated by operations in Ukraine and will briefly assess the significance of these developments for Western security policy.Defining Hybrid WarfareNot surprisingly, there are many definitions of warfare. The concept has been delineated in different, if related, ways and these definitions have evolved in a relatively short period of time. Defining warfare is not just an academic exercise. The way the term is defined may determine how states perceive and respond to threats and which government agencies are involved in countering them.One approach to warfare takes an historical perspective. This defines the term simply as the concurrent use of both conventional and irregular forces in the same military campaign. Military historian Peter R. Mansoor, for example, defines warfare as conflict involving a combination of conventional military forces and irregulars (guerrillas, insurgents, and terrorists), which could include both state and non-state actors, aimed at achieving a common political purpose.2 Viewed from this perspective, warfare is clearly nothing new. There are numerous examples of techniques and approaches at the tactical, operational and strategic levels stretching back at least as far as the Peloponnesian War and the writings of the Chinese philosopher, Sun Tzu, in the fifth century BC. Irregular fighters have proved to be the bane of numerous conventional militaries. Formidable armies such as Napoleon's Grand Armee and Hitler's Wehrmacht struggled to combat irregular fighters who understood and exploited the local human and geographical terrain and targeted vulnerable logistic bases and lines of communication. Over time, guerrilla operations had a significant and lasting impact on the broader conventional military campaigns of which they were part. Recent counter insurgency (COIN) campaigns in Iraq and Afghanistan have once again highlighted the difficulty of defeating de- termined irregular fighters without committing human rights abuses against the local population and consequently undermining domestic and international public support for the campaign.During the 2000s, the use of the term hybrid became a common way to describe contemporary warfare, particularly because of the increasing sophistication and lethality of violent non-state actors and the growing potential of cyber warfare. Although there was no agreement that this necessarily constituted a new form of warfare,3 definitions of warfare emphasized the blending of conventional and irregular approaches across the full spectrum of conflict. For example, in 2007 Frank G. Floffman, a leading analyst of the concept, defined warfare as Threats that incorporate a full range of different modes of warfare including conventional capabilities, irregular tactics and formations, terrorist acts including indiscriminate violence and coercion, and criminal disorder, conducted by both sides and a variety of non-state actors. …