The U.S. military is just another in the long history of conventional militaries struggling to understand and deal with guerrilla or irregular warfare. Traditionally, the U.S. military has fought large scale, conventional conflicts against an enemy in uniform that fought in a manner similar to U.S. forces. With the exception of the Indian Wars of the 19th century, the U.S. military had little experience fighting guerilla wars until Vietnam and avoided them after 1975 until the conflicts in Iraq and Afghanistan. However, not only has irregular warfare become prevalent again, it is morphing into a new form of warfare that combines conventional and irregular warfare into an even more challenging mode of combat called “hybrid warfare.”
One of the principle reasons the U.S. continues to face a challenge in fighting guerrilla, irregular or terrorist forces is that counter-insurgency (COIN) or counter-terrorism operations are about more than traditional kinetic combat- finding, fixing, and using firepower to destroy enemy forces. COIN operations are also about diplomacy, intelligence, propaganda or information operations and even economics and infrastructure building, all of which usually fall into military purview as the only force able to conduct complex operations while maintaining the ability to wage combat. 
Warfare in the 21st century will continue this divergence from traditional conventional combat waged by large mechanized forces of tanks, artillery, and infantry. Recent experiences by the ground forces of Israel and the United States illustrate how new adversaries are developing innovations in tactics and strategy to negate traditional Western conventional military superiority. This has presented a severe challenge to the U.S. and our allies as these traditional notions of conventional, irregular, guerilla, terrorism and criminal activity continue to merge in the primordial soup of failed states in the Third World.
This new type of warfare, labeled “hybrid warfare” by some analysts and defense pundits has created a great deal of uncertainty on the role of conventional military forces and the operational level of war. Since the September 11 attacks, and particularly after the invasion and insurgency in Iraq, the concepts of asymmetric, compound and hybrid warfare have been confused and comingled. The best definition of hybrid warfare combines elements of all of these modes of combat and expands the political use of violence beyond traditional military methods: “Hybrid Wars incorporate a range of different modes of warfare including conventional capabilities, irregular tactics and formations, terrorist acts, including indiscriminate violence and coercion and criminal disorder.” 
Hybrid warfare is often confused with “Fourth-generation” warfare (4GW) which is primarily an insurgency/terrorism mode of war that evolved from the communist guerilla strategy and tactics of Mao Zedong and Ho Chi Minh. 4GW fighters have built upon these concepts of guerilla conflict to incorporate new technologies such as the internet and global media. Hybrid warfare incorporates many of these 4GW tools into a larger construct that includes more conventionally trained and equipped forces and even a functioning legitimate political organization to tie all of these elements of political violence into a coherent and simultaneous strategic and operational plan. 
Even the attempt to differentiate warfare by technology using the construct of generational warfare is becoming meaningless against well-executed asymmetric warfare. Suicide car bombs can be just as effective as tanks and artillery at destroying a building. More importantly, future insurgent and non-state groups will have no compunction about using non-combatants as defenses against Western militaries reluctant to use massive firepower in the face of unbalanced media coverage. For better or worse, Western militaries are held to tighter rules of engagement, which their opponents either blithely ignore or actively circumvent to attack the will of Western societies. The fact that asymmetric opponents are willing to wage unlimited, no-quarter warfare has not been completely understood by Western militaries and is really incomprehensible to modern Western society. Moreover, asymmetric warfare will also bring asymmetric measures of victory, usually to the advantage of the weaker side. Just as powers that wage counter-insurgencies are considered to be losing if they are not winning, in the future, non-state actors and terrorist groups waging asymmetric warfare can “win” a war by simply not being completely annihilated by their opponent while providing propaganda videos to the internet. 
 David Kilcullen, Counterinsurgency (New York, NY: Oxford University Press, 2010), 1-13.
 Frank Hoffman, Conflict in the 21st Century: The Rise of Hybrid Wars (Arlington, VA: Potomac Institute for Policy Studies, 2007), 29.
 Thomas Hammes, The Sling and the Stone: On War in the 21st Century (St Paul, MN: Zenith Press, 2004), 207-223.
 Robert Cassidy, "Why Great Powers Fight Small Wars Badly," Military Review, Sept-Oct 2000: 41-55.