Warlords are individuals who exercise military, economic, and political control over a region, often one without a strong central or national government, typically through informal control over local armed forces. Warlords have existed throughout much of history, albeit in a variety of different capacities within the political, economic, and social structure of states or ungoverned territories. The term is often applied in the context of China around the end of the Qing dynasty, especially during the Warlord Era. The term may also be used for a supreme military leader.

The first appearance of the word "warlord" dates to 1856, when used by American philosopher and poet Ralph Waldo Emerson in a highly critical essay on the aristocracy in England, "Piracy and war gave place to trade, politics and letters; the 'war-lords to the law-lord; the privilege was kept, whilst the means of obtaining it were changed."

During the First World War, the term appeared in China as Junfa (軍閥), taken from the Japanese gunbatsu. It was not widely used until the 1920s, when it was used to describe the chaos after 1918, when provincial military leaders took local control and launched the period that would come to be known in China as the Warlord Era. In China, Junfa is applied retroactively to describe the leaders of regional armies who threatened or used violence to expand their rule, including those who rose to lead and unify kingdoms.

The other major consideration in categorizing warlords is through the lens of history. Warlordism was a widespread, dominant political framework that ordered many of the world's societies until the modern state became globally ubiquitous. Often warlord governance in pre-modern state history was constructed along tribal or kinship lines and was congruent with early perception of "nation". In colonial empires warlords served in both cooperative political capacities and as leaders of rebellions. In modern states the presence of warlords is often seen as an indicator of state weakness or failure. American historian David G. Herrmann noted, "Warlordism is the default condition of humanity."

Economist Stergios Skaperdas views warlordism as a default—albeit inefficient—competitive economic model that emerges in states where state capacity is low, but that innately evolves into an institution governing political order that uses violence or the threat of it to secure its access to "rent"-producing resources. It may actually have a stabilizing effect on a region. In both cases there is an inherent inefficiency in the model, as "resources are wasted on unproductive arming and fighting." However, the functionality is often sustainable because it presents citizens with no choice but to accept rent levies in exchange for protection. Charles Tilly, an American political scientist and sociologist, theorized that organized crime can function as a means for war and state making. He argues that the monopoly of crime by the state—in this case being the warlords—is in order to receive protection from external rivals as well as internal political rivals.

Political scientist Jesse Driscoll uses the term "redistribution politics" to classify the bargaining process between warlords and the regime in states where cooperative warlord politics prevails, and when that bargaining leads to accords or informal arrangements concerning the extraction of rent—which can refer to natural resources, territory, labor, revenue or privilege. In his study of warlordism in Georgia and Tajikistan, Driscoll cites "land reform, property ownership and transfers, privatization in non-transparent closed-bid settings, complex credit swaps cemented via marriages, money laundering, price-fixing schemes, and bribery", as principal sources of exchange in redistribution politics.

Noted theorist Max Weber suggested that classic feudalism in pre-modern-state Europe was an example of warlordism, as the state regime was unable to "exercise a monopoly on the use of force within its territory" and the monarch relied on the commitment of loyal knights and other nobility to mobilize their private armies in support of the crown for specific military campaigns. As noted French philosopher Alexis de Tocqueville and political scientists such as E. J. Hobsbawm and Theda Skocpol observed in their analyses of the Ancien Régime, the French Revolution and democratization in Europe, that commitment was contingent upon a bargaining process in which the king or queen had to guarantee additional territory, revenue, status or other privileges, meaning that these early European states were weak and the relationship between the crown and feudal lords constituted the form of interdependent warlordism known as cooperative warlord politics.

Under the feudal system of Europe, nobility—whether feudal lords, knights, princes or barons—were warlords in that they served as regional leaders who exercised military, economic and political control over subnational territories and maintained private armies to maintain that status. While their political power to exercise social order, welfare and regional defense within their territory was derived from hereditary rights or edicts from the monarch, their military strength afforded them independence and strength to negotiate for privileges. Should the feudal lord or other noble withdraw his support from the king, either in rebellion or to form an alliance with a rival kingdom, that feudal lord or noble was now ascribing to the political order of ungoverned warlordism.