Causes of war across the eras
Why Do Wars Begin?
Wars rarely have a single cause. Across the centuries, Britain has been drawn into conflicts by a shifting mixture of factors: religious conviction, political rivalry, national pride, and competing ideologies. Understanding these causes is central to the thematic study of warfare — and to writing strong analytical essays that trace change and continuity across the medieval, early modern and modern periods.
Four recurring causes of war in British history are religion, politics, nationalism and ideology. No single factor dominates across all eras — the relative importance of each shifts significantly from medieval times to the present day.
Religion: The Crusades
The Crusades are the clearest example of religion as a cause of war in the medieval period. Pope Urban II called the First Crusade in 1095, urging Christian knights to travel to Jerusalem and wrest it from Muslim rule. His appeal combined genuine religious fervour with the promise of spiritual reward: those who died on Crusade were offered absolution from their sins.
English and Norman knights participated in multiple Crusades across the twelfth and thirteenth centuries. King Richard I of England — known as “the Lionheart” — led the Third Crusade (1189–1192) and came close to recapturing Jerusalem. Religion here acted as both the stated motivation and the organising framework that could mobilise armies across national boundaries. The concept of a ‘just war’ fought in God’s name legitimised warfare in a way that was specific to the medieval worldview.
Religion did not entirely disappear as a cause of conflict after the medieval period. The English Reformation of the 1530s generated decades of religious tension that fed into later conflicts — including the English Civil War, where disputes about the Church of England’s governance were part of the broader breakdown between Crown and Parliament. However, by the twentieth century, religion had largely been displaced by nationalism and ideology as the primary driver of large-scale war. This shift from religion to secular causes is itself a key pattern of change in British warfare.
Politics: The English Civil War
The English Civil War (1642–1651) illustrates politics as a cause of conflict. The war grew from a profound constitutional crisis: King Charles I insisted on the divine right of kings and attempted to rule without Parliament for eleven years (the “Personal Rule”, 1629–1640). When he needed money to suppress a Scottish rebellion, he was forced to recall Parliament, which used its renewed power to challenge royal authority.
The conflict was not simply about who held power; it involved competing visions of government. Divine right of kings was Charles’s guiding principle. Parliament countered with arguments for constitutional limits on royal power. The Civil War ended with the execution of Charles I in 1649, the only time an English monarch was tried and killed by Parliament.
Nationalism and Expansionism: The Second World War
Fascism and aggressive nationalism drove Germany’s path to war in the 1930s. Adolf Hitler came to power in 1933 promising to restore German greatness after the humiliations of the Treaty of Versailles (1919), which had stripped Germany of territory, imposed heavy reparations, and limited its armed forces. Nazi ideology combined nationalism with racial ideology, the pursuit of Lebensraum (living space) in Eastern Europe, and the desire to reverse the post-1919 settlement.
For Britain, the Second World War (1939–1945) began as a response to German expansionism — specifically the invasion of Poland in September 1939. Nationalism acted as a cause in two directions: German nationalism provided the aggressive drive for territorial expansion, while British national identity — a belief in defending freedom and honouring treaty commitments — provided the motivation for resistance. This contrast shows how nationalism could simultaneously drive aggression and resistance in the same conflict.
German nationalism and Nazi expansionism were the primary cause of the Second World War in Europe. Britain entered the war in September 1939 to honour its guarantee to Poland, which Germany had invaded. The conflict shows that nationalism can function as both an aggressive and a defensive cause of war.
Ideology: The Cold War
The Cold War (c.1947–1991) was fundamentally an ideological conflict. The United States and its allies, including Britain, championed liberal democracy and free-market capitalism. The Soviet Union promoted communism — a system in which the state owned the means of production and individual political freedoms were subordinated to collective goals. These two systems were seen as mutually incompatible.
Britain’s involvement in the Cold War included membership of NATO (formed 1949), the development of its own nuclear deterrent, and participation in proxy conflicts such as the Korean War (1950–1953). Unlike previous British wars, the Cold War never became a direct armed conflict between the superpowers, but the ideological rivalry shaped British military policy, defence spending and alliance commitments for four decades. Ideology here was not merely a background factor; it was the primary organising principle of the entire conflict.
Across the three eras, there are clear patterns of change and some continuity. In the medieval period, religion dominated as the publicly stated cause of war. In the early modern period (roughly 1500–1800), political disputes — about the nature of government, constitutional authority, and dynastic succession — became primary. In the modern period (1800–present), nationalism and ideology have been the most powerful causes. One element of continuity is that economic interest has always played a background role — the Crusades offered loot and land, the English Civil War partly concerned taxation rights, and twentieth-century conflicts involved access to resources and markets. When writing a thematic change essay, aim to trace this pattern across all three eras rather than treating each as a separate topic.