While the dividing line between modern and pre-modern conflicts and wars cannot be strictly defined, however, a combination of technological, political, social, and conceptual changes that occurred over time led from one to the other. Historians and military theorists often use several key developments as markers of the transition from pre-modern to modern warfare.

The Peace of Westphalia (1648), which ended the Thirty Years' War in Europe, is considered by some scholars as a foundational moment in the establishment of the modern state system that emphasized the principles of sovereignty and non-interference in the internal affairs of states, laying the groundwork for the modern international order. Hence, we use that in our classification, too, and treat all conflicts, battles and wars prior to 1648 as "pre-modern".

The second period in our classification system is the transition period from "pre-modern" to "modern" warfare, that is, from 1648 to World War I at the beginning of the twentieth century. This period saw the Industrial Revolution (late 18th to early 19th Centuries), which had profound effects on warfare, including the mass production of weapons, the use of steam-powered ships and trains for military logistics, and later, the introduction of mechanized warfare. The French Revolution and Napoleonic Wars in this era introduced the concept of total war and national conscription, significantly changing the scale and nature of conflict. The mass mobilization of populations and the ideological motivations behind the wars were indicative of modern warfare. The American Civil War (1861-1865), sometimes considered the first modern war, saw the use of advanced military technology (e.g., ironclad ships, railroads for troop movement, telegraph for communication, and more sophisticated firearms), and concepts of total war.

The current period of "modern war" in our classification system commences with World War I (1914-1918), which many historians view as the definitive advent of modern warfare on a global scale, characterized by the extensive use of machine guns, tanks, airplanes, chemical weapons, and trench warfare, along with the involvement of industrialized nation-states mobilizing millions of troops and civilians.

Using these markers, we proceed to classify our compendium into three periods at the top.