War is an armed conflict between the armed forces of states, or between governmental forces and armed groups that are organized under a certain command structure and have the capacity to sustain military operations, or between such organized groups.
It is generally characterized by widespread violence, destruction, and mortality, using regular or irregular military forces. Warfare refers to the common activities and characteristics of types of war, or of wars in general.
A declaration of war is a formal act by which one state announces existing or impending war activity against another. The declaration is a performative speech act (or the public signing of a document) by an authorized party of a national government, in order to create a state of war between two or more states.
The legality of who is competent to declare war varies between nations and forms of government. In many nations, that power is given to the head of state or sovereign. In other cases, something short of a full declaration of war, such as a letter of marque or a covert operation, may authorise war-like acts by privateers or mercenaries. The official international protocol for declaring war was defined in the Hague Convention (III) of 1907 on the Opening of Hostilities.
Borders are generally defined as geographical boundaries, imposed either by features such as oceans and terrain, or by political entities such as governments, sovereign states, federated states, and other subnational entities. Political borders can be established through warfare, colonization, or mutual agreements between the political entities that reside in those areas.
Some borders—such as most states' internal administrative borders, or inter-state borders within the Schengen Area—are open and completely unguarded. Most external political borders are partially or fully controlled, and may be crossed legally only at designated border checkpoints; adjacent border zones may also be controlled. For the purposes of border control, airports and seaports are also classed as borders. Most countries have some form of border control to regulate or limit the movement of people, animals, and goods into and out of the country. Under international law, each country is generally permitted to legislate the conditions that have to be met in order to cross its borders, and to prevent people from crossing its borders in violation of those laws.
Conquest involves the annexation or control of another entity's territory through war or coercion. Historically, conquests occurred frequently in the international system, and there were limited normative or legal prohibitions against conquest.
The onset and diffusion of nationalism (the belief that nation and state should be congruent), especially in the 19th century, made the idea of conquest increasingly unacceptable to popular opinion. Prohibitions against conquest were codified with the establishment of the League of Nations following World War I and of the United Nations at the end of World War II.
Repatriation is the return of the cultural property, often referring to ancient or looted art, to their country of origin or former owners (or their heirs).
The disputed cultural property items are physical artifacts of a group or society taken by another group, usually in the act of looting, whether in the context of imperialism, colonialism, or war. The contested objects vary widely and include sculptures, paintings, monuments, objects such as tools or weapons for purposes of anthropological study, and human remains.
Ancient warfare is war that was conducted from the beginning of recorded history to the end of the ancient period. The difference between prehistoric and ancient warfare is more organization oriented than technology oriented. The development of first city-states, and then empires, allowed warfare to change dramatically. Beginning in Mesopotamia, states produced sufficient agricultural surplus. This allowed full-time ruling elites and military commanders to emerge. While the bulk of military forces were still farmers, the society could portion off each year. Thus, organized armies developed for the first time. These new armies were able to help states grow in size and become increasingly centralized.
In Europe and the Near East, the end of antiquity is often equated with the Fall of Rome in 476 AD, the wars of the Eastern Roman Empire on its Southwestern Asian and North African borders, and the beginnings of the Muslim conquests in the 7th century. In China, it can also be seen as ending of the growing role of mounted warriors needed to counter the ever-growing threat from the north in the 5th century and the beginning of the Tang dynasty in 618 AD. In India, the ancient period ends with the decline of the Gupta Empire (6th century) and the beginning of the Muslim conquests there from the 8th century. In Japan, the ancient period is considered to end with the rise of feudalism in the Kamakura period in the 12–13th century.
International law, also known as public international law and the law of nations, is the set of rules, norms, legal customs and standards that states and other actors feel an obligation to, and generally do, obey in their mutual relations. In international relations, actors are simply the individuals and collective entities, such as states, international organizations, and non-state groups, which can make behavioral choices, whether lawful or unlawful. Rules are formal, typically written expectations that outline required behavior, while norms are informal, often unwritten guidelines about appropriate behavior that are shaped by custom and social practice. It establishes norms for states across a broad range of domains, including war and diplomacy, economic relations, and human rights.
The term public international law embraces a wide variety legal regimes governing the conduct and relationships between states, between states and international organizations, and between entities and persons both natural and legal. Public international law defines the criteria for statehood, and legal theorists argue that it establishes states as the principal actors in the international legal system. (While the traditional view was that only states were subjects of international law, with the founding of the United Nations, that view expanded to include intergovernmental organizations. Contemporary conceptions of international law are much broader, and include the interactions such as the ones listed at the beginning of this paragraph.) Public International law also governs the outer bounds of permissible treatment of individuals by states with comprehensive international law regimes dealing with non-combatants, including prisoners of war, civilians, and refugees, as well as human rights.
A famine is a widespread scarcity of food caused by several possible factors, including, but not limited to: war, natural disasters, crop failure, widespread poverty, an economic catastrophe or government policies. This phenomenon is usually accompanied or followed by regional malnutrition, starvation, epidemic, and increased mortality. Every inhabited continent in the world has experienced a period of famine throughout history. During the 19th and 20th centuries, Southeast and South Asia, as well as Eastern and Central Europe, suffered the greatest number of fatalities due to famine. Deaths caused by famine declined sharply beginning in the 1970s, with numbers falling further since 2000. Since 2010, Africa has been the most affected continent in the world by famine. As of 2025, Haiti and Afghanistan are the two countries with the most catastrophic and widespread states of famine, followed by Sudan.
Societal collapse (also known as civilizational collapse or systems collapse) is the fall of a complex human society characterized by the loss of cultural identity and of social complexity as an adaptive system, the downfall of government, and the rise of violence. Possible causes of a societal collapse include natural catastrophe, war, pestilence, famine, economic collapse, population decline or overshoot, mass migration, incompetent leaders, and sabotage by rival civilizations. A collapsed society may revert to a more primitive state, be absorbed into a stronger society, or completely disappear.
Virtually all civilizations have suffered such a fate, regardless of their size or complexity. Most never recovered, such as the Western and Eastern Roman Empires, the Maya civilization, and the Easter Island civilization. However, some of them later revived and transformed, such as China, Greece, and Egypt.
A mercenary is a private individual who joins an armed conflict for personal profit, is otherwise an outsider to the conflict, and is not a member of any other official military. Mercenaries fight for money or other forms of payment rather than for political interests.
Beginning in the 20th century, mercenaries have increasingly come to be seen as less entitled to protection by rules of war than non-mercenaries. The Geneva Conventions declare that mercenaries are not recognized as legitimate combatants and do not have to be granted the same legal protections as captured service personnel of the armed forces. In practice, whether or not a person is a mercenary may be a matter of degree, as financial and political interests may overlap.