foreign troops. The Tang economy was devastated as the northern regions, the mainstay of economic activity, were destroyed. The once populous cities of Luoyang and Chang'an were reduced to ruins. After the war, the Tang central government never recovered its former power, and local generals gradually became independent of Tang rule. These generals wielded enormous power, passed on their titles by heredity, collected taxes, and raised their own armies. Although most of these jiedushi were quelled by the early 9th century, the jiedushi of Hebei, many of them former officers of An Lushan, retained their independence. The weakened Tang government was forced to abolish many of its regulations and interventions after the rebellion; this unintentionally stimulated trade and commerce in Tang China, which reached an apogee in the early 9th century. The economic centre of China shifted southwards because the north had been devastated. The equal-field system, which had formed the basis of agriculture for the past two and a half centuries, began to collapse after the An Shi Rebellion. The equal-field system had relied on the state having large amounts of land, but state landholdings had decreased as they were privatised or granted to peasants. Landowners perpetually enlarging their estates exacerbated the collapse. The Fubing system, in which soldiers served the army and farmed on equal-field land was abolished and replaced with the Mubing system, which relied on a volunteer and standing army. In 780, the Tang government discontinued the equal field system and reorganised the tax system, collecting taxes based on property value twice a year, in spring and autumn. Woodblock printing began to develop during the 8th and 9th centuries. This grew out of the huge paper industry that had emerged since the Han. Woodblock printing allowed the rapid production of many books, and increased the speed at which knowledge spread. The first book to be printed in this manner, with a production date, was the Jin Gang Jin, a Buddhist text printed in 868, but the hand-copying of manuscripts still remained much more important for centuries to come. During the later years of the Tang dynasty, overseas trade also began with East Africa, India, and the Middle East. Although the economy recovered during the 9th century, the central government was weakened. The most pressing issue was the government's salt monopoly, which raised revenue after the equal-field system collapsed and the government could no longer collect land