Italy’s thriving economy soon confronted severe challenges. Among these, first, were famines, which affected most of Italy in the years 1339–40, 1346–47, 1352–53, and 1374–75, and a general expansion and intensification of war compounded these catastrophes. The 13th century saw the diffusion of the crossbow, whose bolt far surpassed the arrow of the longbow in its power to penetrate. The crossbow obliged mounted knights to adopt heavier armour for better protection. Hence arose the need for stronger and more-numerous horses. Such technical developments began to make the practice of warfare much more expensive and professional, and in these circumstances mercenary troops came increasingly to supplement and then often to replace the old citizen militias. In the 14th century, Italian states raised these troops in ever larger numbers not by hiring individuals but by drawing up a condotta (contract) with a condottiere (contractor), who would engage to bring a band of up to several thousand soldiers in time of war to the aid of a commune or kingdom.

Given the difficulties of securing political control over Italian military leaders (who might, it was feared, take over the state), it became common, beginning in the 1330s, to negotiate with non-Italian condottieri. Their forces rapidly grew to immense size. In the 1350s “The Great Company,” founded by Werner of Urslingen, comprised some 10,000 troops and 20,000 camp followers and had its own government, consultative council, bureaucracy, and foreign policy. Throughout the 1360s and ’70s these “mobile states”—for example, the companies of the Englishman Sir John Hawkwood and the Germans Albrecht Sterz and Hannekin Baumgarten—dominated war in Italy, and in times of peace they were all too likely to subject their former employers to a variety of blackmailing threats.

These changes in the practice of war went hand in hand with a considerable expansion in the power of governments. The weak, decentralized communes of the 13th century, with comparatively primitive administration and very light taxation, gave way in the 14th century to republics and signorie with much stronger political control and exclusive new means of fiscal exploitation. States raised revenues through property taxes, gabelle (e.g., taxes on contracts, sales, transport of goods into and out of town), and forced loans (prestanze), while they developed sophisticated measures, including the consolidation of state debts into a form of national debt, to service long-term deficit financing. At Florence, for example, where from 1345 state debtors were issued securities at 5 percent interest, negotiable in the open market, revenues rose from around 130,000 florins in the 1320s to more than 400,000 florins in the 1360s.

Such innovations—fruit of the interrelated needs of food provision, war, and taxation—brought about considerable growth in bureaucratic institutions and in the number of administrative officials. At the same time, however, these innovations allowed war to be waged on a larger scale, and states increasingly diverted productive wealth into war. That is, these innovations helped cause the setbacks that occurred in many sectors of the economy during the 1340s. In that decade, with trade already disrupted by the beginning of the Hundred Years’ War in France, the overextension of Italian (particularly Florentine) banks became clear. In 1343 the Peruzzi company collapsed, in 1345 the Acciaiuoli, and in 1346 the Bardi.

Still more disastrous was the arrival from the east of the Black Death. Galleys and cogs brought the plague in its bubonic and pneumonic forms to Messina in early October 1347. By January 1348 it had reached Genoa and Pisa, by February Venice. From these ports it spread throughout the peninsula and on to the rest of Europe. Estimates of the death toll vary between one-third and one-half of the population. Yet the effects were not confined to 1348, as plague was henceforth rooted in Italy. Although slackening in its power and appearing more sporadically, the disease returned to many parts of the peninsula, in both town and countryside, in 1361–62, 1363, 1371, 1373–74, 1382–83, 1398–1400, 1407, and 1410–12. Thereafter it continued as a town disease in individual, sporadic, but continually threatening assaults up to the 18th century.