Pirenne made a lasting contribution to the study of cities that was a controversial interpretation of the end of Roman civilization and the rebirth of medieval urban culture.

[2] 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.

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.

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.

[8] 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.

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.

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.

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.

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.

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 in northern Europe after the 7th century; writing reverted to using parchment, indicating its economic isolation.

[13][14] It continues to inform historical discussion in the 21st century, with more recent debate focusing on whether later archaeological discoveries refute the thesis or demonstrate its fundamental viability.

Belgium as an independent nation-state had appeared in 1830 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 northern France.

Pirenne's Histoire de Belgique, published in seven volumes from 1899–1932, stressed how traditional and economic forces had drawn the Flemish and Walloons together.

Pirenne, inspired by patriotic nationalism, presupposed the existence of a unified "Belgian civilisation" – in a social, political, and ethnic sense – which predated its 1830 independence by centuries.

His thesis 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 favour.

Pirenne was 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 Muslim world, and opened up sea routes to the Orient.