See studies by A. F. Havighurst, ed. (rev. ed. 1969) and B. P. Lyon (1972).
Henri Pirenne's reputation today rests on three contributions to European history. First, what has become known as the Pirenne Thesis, concerning origins of the Middle Ages in reactive state formation and shifts in trade; secondly, for a distinctive view of Belgium's medieval history; and, thirdly, for his model on the development of the medieval city.
In brief, Pirenne's Thesis 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 a suburb 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 bought 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. Pirenne 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 equate 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, that the Roman way of doing things did not fundamentally change in the time immediately after the "fall" of Rome. Barbarians came to Rome not to destroy it, but to take part in its benefits; they tried to preserve the Roman way of life.
According to Pirenne the real break in Roman history occurred in the 7th 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 Europe, cutting the continent 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 Europe had become entirely agrarian at a subsistence level, with no long-distance trade. Pirenne says "Without Islam, the Frankish Empire would have probably never existed, and Charlemagne, without Muhammad, would be inconceivable".
Pirenne used quantitative methods in relation to currency in support of his thesis. Much of his argument builds upon the disappearance of items from Europe, items that had to come from outside Europe. 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.
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 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 centuries, Europe reclaimed control of the Mediterranean from the Moslem 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.
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 and incomplete work which Pirenne wrote while imprisoned in Germany during World War I. It was published by his son in 1936. An 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 is not known. What is known is that Pirenne was questioned by German occupiers on March 18, 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. While being questioned, the German officer asked Pirenne 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 August 3, 1914," the date of the German invasion of Belgium, part of Germany's war plan to conquer France.
Henri Pirenne was held in Crefeld, then Holzminden, then Jena where he was interned from August 24, 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 in its own right.