For about 300 or 350 years most conflicts in the world, or at least the major ones, were between and among nation-states, that is one country fighting another or several fighting each other. More often than not these conflicts were about boundaries, territory, aggrieved minorities, religious or ethnic friction, or simply raw power.
Conventional nation-state wars evolved into large armies wearing national uniforms, employing ever more sophisticated large weapons, often meeting in decisive battles in more or less open fields. These conflicts created their own rules embodied in international law and Geneva conventions.
Beginning sometime in the post-World War II time of colonial disintegration, so-called wars of national liberation sprang up, one country trying to rid itself of an occupying power. This produced guerilla tactics—non-uniformed, indigenous forces using light weapons, hit-and-run methods, and often hitting civilian targets. These kinds of conflicts proliferated when the bi-polar lid of the Cold War was lifted. We experienced this unconventional warfare in Vietnam as the Soviets did (and now the U.S. does) in Afghanistan.
Largely under the threat of weapons of mass destruction, nation-state wars are declining. But irregular, unconventional conflicts are expanding. History may record its inaugural date as September 11, 2001, but its roots are at least a half-century older.