A skilled longbowman could shoot about 12 shots per minute. This rate of fire was far superior to competing weapons like the crossbow or early gunpowder weapons. The nearest competitor to the longbow was the much more expensive crossbow, used often by urban militias and mercenary forces. The crossbow had greater penetrating power and did not require the extended years of training. However, it lacked the rate of fire of the longbow. At Crécy and Agincourt bowmen unleashed clouds of arrows into the ranks of knights. At Crécy, even 5,000 Genoese crossbowmen could not dislodge them from their hill. At Agincourt, thousands of French knights were brought down by armour-piercing bodkin point arrows and horse-maiming broadheads. Longbowmen decimated an entire generation of the French nobility. In 1326 the earliest known European picture of a gun appeared in a manuscript by Walter de Milemete. In 1350, Petrarch wrote that the presence of cannons on the battlefield was 'as common and familiar as other kinds of arms'. Early artillery played a limited role in the Hundred Years' War, and it became indispensable in the Italian Wars of 1494–1559, marking the beginning of early modern warfare. Charles VIII, during his invasion of Italy, brought with him the first truly mobile siege train: culverins and bombards mounted on wheeled carriages, which could be deployed against an enemy stronghold immediately after arrival. The initial Muslim conquests began in the 7th century after the death of the Islamic prophet Muhammad, and were marked by a century of rapid Arab expansion beyond the Arabian Peninsula under the Rashidun and Umayyad Caliphates. Under the Rashidun, the Arabs conquered the Persian Empire, along with Roman Syria and Roman Egypt during the Byzantine-Arab Wars, all within just seven years from 633 to 640. Under the Umayyads, the Arabs annexed North Africa and southern Italy from the Romans and the Arab Empire soon stretched from parts of the Indian subcontinent, across Central Asia, the Middle East, North Africa, and southern Italy, to the Iberian Peninsula and the Pyrenees. The early Arab army mainly consisted of camel-mounted infantry, alongside a few Bedouin cavalry. Constantly outnumbered by their opponent, they did, however, possess the advantage of strategic mobility, their camel-borne nature allowing them to constantly outmanoeuvre larger Byzantine and Sassanid armies to take prime defensive positions.