Henri Pirenne (23 December 1862, Verviers - 25 October 1935, Uccle) was a leading Belgian historian. A medievalist of Walloon descent, he wrote a masterful multivolume history of Belgium in French and became a national hero. Historians continue to debate his influential theories about medieval history.

He also became prominent in the nonviolent resistance to the Germans who occupied Belgium in World War I.

Henri Pirenne's reputation today rests on three contributions to European history: for what has become known as the Pirenne Thesis, concerning origins of the Middle Ages in reactive state formation and shifts in trade; for a distinctive view of Belgium's medieval history; and for his model of the development of the medieval city.

Pirenne argued that profound, long term social, economic, cultural, and religious movements resulted from profound underlying causes, and this attitude influenced Marc Bloch and the outlook of the French Annales School of social history.

At the University of Liège he was a student of Godefroid Kurth (1847 - 1916). He became Professor of History at the University of Ghent in 1886, a post he held until the end of his teaching career, in 1930. After the Great War he was the most prominent and influential historian in Belgium, receiving numerous honors and committee assignments. Pirenne was a close friend of German historian Karl Lamprecht (1856 - 1915), until they broke during the war when Lamprecht headed a mission to invite Belgians to collaborate with Germany's long term goals.

Pirenne was a leader of Belgian passive resistance in the war; the Germans imprisoned him in Germany (1916 – 18) as a civilian prisoner of war. He learned Russian from other prisoners and composed a history of Europe, written in isolation from libraries and archives. After the war, he reflected the widespread disillusionment in Belgium with German culture, while taking a nuanced position which allowed him to criticize German nationalism without excluding German works from the scholarly canon. His earlier belief in the inevitable progress of humanity collapsed, so he began to accept chance or the fortuitous in history and came to acknowledge the significance of single great individuals at certain points in history.

Henri Pirenne first expressed ideas on the formation of European towns in articles of 1895; he further developed the idea for the Pirenne Thesis while imprisoned in Germany during World War I. He subsequently published it in a series of papers from 1922 to 1923 and spent the rest of his life refining the thesis with supporting evidence. The most famous expositions appear in Medieval Cities: Their Origins and the Revival of Trade (1927, based on a series of lectures of 1922) and in his posthumous Mohammed and Charlemagne (1937), published from Pirenne's first draft.

In brief, the Pirenne Thesis, an early essay in economic history diverging from the narrative history of the 19th century, notes that in the ninth century long distance trading was at a low ebb; the only settlements that were not purely agricultural were the ecclesiastical, military and administrative centres that served the feudal ruling classes as fortresses, episcopal seats, abbeys and occasional royal residences of the peripatetic palatium. When trade revived in the late tenth and eleventh centuries, merchants and artisans were drawn to the existing centres, forming suburbs in which trade and manufactures were concentrated. These were "new men" outside the feudal structure, living on the peripheries of the established order. The feudal core remained static and inert. A time came when the developing merchant class was strong enough to throw off feudal obligations or to buy out the prerogatives of the old order, which Pirenne contrasted with the new element in numerous ways. The leaders among the mercantile class formed a bourgeois patriciate, in whose hands economic and political power came to be concentrated.

Pirenne's thesis takes as axiomatic that the natural interests of the feudal nobility and of the urban patriciate, which came to well attested frictions in the thirteenth and fourteenth centuries, were in their origins incompatible. This aspect of his thesis has been challenged in detail.

Traditionally, historians have dated the Middle Ages from the fall of the Roman Empire in the 5th century, a theory Edward Gibbon famously put forward in the 18th century, which is inexorably linked to the supposition of a Roman "decline" from a previous classic ideal. Pirenne postponed the demise of classical civilization to the seventh century. He challenged the notion that Germanic barbarians had caused the Roman Empire to end and he challenged the notion that the end of the Roman Empire should be equated with the end of the office of emperor in Europe, which occurred in 476. He pointed out the essential continuity of the economy of the Roman Mediterranean even after the barbarian invasions, and that the Roman way of doing things did not fundamentally change in the time immediately after the "fall" of Rome. Barbarian Goths came to Rome not to destroy it, but to take part in its benefits; they tried to preserve the Roman way of life. The more recent formulation of a historical period characterized as "Late Antiquity" emphasizes the transformations of ancient to medieval worlds within a cultural continuity, and European archaeology of the first millennium, purposefully undertaken in the later 20th century, even extends the continuity in material culture and patterns of settlement below the political overlay as reaching as late as the eleventh century.

According to Pirenne the real break in Roman history occurred in the 8th century as a result of Arab expansion. Islamic conquest of the area of today's south-eastern Turkey, Syria, Palestine, North Africa, Spain and Portugal ruptured economic ties to western Europe, cutting the region off from trade and turning it into a stagnant backwater, with wealth flowing out in the form of raw resources and nothing coming back. This began a steady decline and impoverishment so that by the time of Charlemagne western Europe had become almost entirely agrarian at a subsistence level, with no long distance trade.

Pirenne used statistical data regarding money in support of his thesis. Much of his argument builds upon the disappearance from western Europe of items that had to come from outside. For example, the minting of gold coins north of the Alps stopped after the 7th century, indicating a loss of access to wealthier parts of the world. Papyrus, made only in Egypt, no longer appeared north of the Alps after the 7th century: writing reverted to using animal skins, indicating an isolation from wealthier areas.

In a summary, he famously said, "Without Islam, the Frankish Empire would have probably never existed, and Charlemagne, without Muhammad, would be inconceivable." That is, he rejected the notion that barbarian invasions in the 4th and 5th centuries caused the collapse of the Roman Empire. Instead, the Muslim conquest of north Africa made the Mediterranean a barrier, cutting western Europe off from the east, enabling the Carolingians, especially Charlemagne, to create a new, distinctly western form of government.

Pirenne's Thesis has not entirely convinced all historians of the period. One does not have to entirely accept or deny his theory. It has provided useful tools for understanding the period of the Early Middle Ages, and a valuable example of how periodization schemes are provisional, never axiomatic.

Pirenne's other major idea concerned the nature of medieval Belgium. Belgium as an independent nation state had appeared only a generation before Pirenne's birth; throughout Western history, its fortunes had been tied up with the Low Countries, which now include the Netherlands, Luxembourg and parts of north-east France. Furthermore, Belgium lies athwart the great linguistic divide between French and Dutch. The unity of the country might appear accidental, something which Pirenne sought to disprove in his History of Belgium (1899 - 1932). His ideas here, promoting a kind of Belgian nationalism, have also proved controversial, with many historians preferring to stress the economic unity of the Low Countries as a whole. Henri Pirenne donated the majority of his personal library to the Academia Belgica in Rome. In 1933, he was awarded the Francqui Prize on Human Sciences.

Pirenne's Histoire de Belgique (7 vol., 1899 – 1932) stressed how traditional and economic forces had drawn Flemings and Walloons together. Pirenne, inspired by patriotic nationalism, presupposed a Belgian unity - social, political, and ethnic - which predated its 1830 independence by centuries. Although a liberal himself, he wrote his seven volume history with such a masterly balance that Catholics, liberals and socialists could quote from it with equal respect in their newspapers or sometimes even in their political gatherings.

Pirenne's history remains crucial to the understanding of Belgium's past, but his notion of a continuity of Belgian civilization forming the basis of political unity has lost favor. Many Belgian scholars feel that the creation of their country was a historical accident. Pirenne's argument that the long Spanish rule in the Low Countries had little continuing cultural impact has likewise fallen, in the face of new research since 1970 in the fields of cultural, military, economic, and political history.

Pirenne is also the author of Medieval Cities: Their Origins and the Revival of Trade (1927), a book based on lectures he delivered in the United States in 1922. In this book he contends that through the period from the tenth to the twelfth century, Europe reclaimed control of the Mediterranean from the Muslim world, and opened up sea routes to the Orient. This allowed the formation of a merchant / middle class and the development of that class's characteristic abode, the city.

He argued that capitalism originated in Europe's cities, as did democracy. His "Merchant Enterprise School" opposed Marxism but shared many of Marx's ideas on the merchant class. Pirenne's theory of a commercial renaissance in towns in the 11th century remains the standard interpretation.

Pirenne wrote a two volume A History of Europe: From the End of the Roman World in the West to the Beginnings of the Western States, a remarkable but incomplete work which Pirenne wrote while imprisoned in Germany during World War I. It was published by his son in 1936. A translation into English, by Bernard Miall, was first published in Great Britain in 1939 by George Allen and Unwin.

How involved Pirenne was in the Belgian resistance during World War I is not known. What is known is that Pirenne was questioned by German occupiers on 18 March 1916, and subsequently arrested. The occupying army had ordered striking professors at the University of Ghent to continue teaching. Pirenne's son Pierre had been killed in the fighting at the Battle of the Yser in 1914. The German officer questioning Pirenne asked why he insisted on answering in French when it was known that Pirenne spoke excellent German and had done postgraduate studies at Leipzig and Berlin. Pirenne responded: "I have forgotten German since 3 August 1914," the date of the German invasion of Belgium, part of Germany's war plan to defeat France.

Pirenne was held in Crefeld, then in Holzminden, and finally in Jena, where he was interned from 24 August 1916 until the end of the war. He was denied books, but he learned Russian from soldiers captured on the Eastern Front and subsequently read Russian language histories made available to him by Russian prisoners. This gave Pirenne's work a unique perspective. At Jena, he began his history of medieval Europe, starting with the fall of Rome. He wrote completely from memory. Rather than a blow-by-blow chronology of wars, dynasties and incidents, A History of Europe presents a big picture approach to social, political and mercantile trends. It is remarkable not only for its historical insight, but also its objectivity, especially considering the conditions under which it was written.

At the conclusion of the war, Henry Pirenne stopped his work on A History of Europe in the middle of the 16th century. He returned home and took up his life. At the time of his death in 1935, Pirenne's son Jacques Pirenne, who had survived the war to become a historian in his own right, discovered the manuscript. He edited the work by inserting dates for which his father was uncertain in parentheses. Jacques wrote a preface explaining its provenance and published it, with the English translation appearing in 1956. It stands as a monumental intellectual achievement.