The Tang dynasty (Chinese: 唐朝) was an imperial dynasty of China preceded by the Sui dynasty and followed by the Five Dynasties and Ten Kingdoms period. It is generally regarded as a high point in Chinese civilization, and a golden age of cosmopolitan culture. Its territory, acquired through the military campaigns of its early rulers, rivaled that of the Han dynasty, and the Tang capital at Chang'an (present-day Xi'an) was the most populous city in the world. The dynasty was founded by the Lǐ family (李), who seized power during the decline and collapse of the Sui Empire. The dynasty was briefly interrupted when Empress Wu Zetian seized the throne, proclaiming the Second Zhou dynasty (690–705) and becoming the only Chinese empress regnant. In two censuses of the 7th and 8th centuries, the Tang records estimated the population by number of registered households at about 50 million people. Yet, even when the central government was breaking down and unable to compile an accurate census of the population in the 9th century, it is estimated that the population had grown by then to about 80 million people. With its large population base, the dynasty was able to raise professional and conscripted armies of hundreds of thousands of troops to contend with nomadic powers in dominating Inner Asia and the lucrative trade routes along the Silk Road. Various kingdoms and states paid tribute to the Tang court, while the Tang also conquered or subdued several regions which it indirectly controlled through a protectorate system. Besides political hegemony, the Tang also exerted a powerful cultural influence over neighboring states such as those in Korea, Japan, and Vietnam. The Tang dynasty was largely a period of progress and stability in the first half of the dynasty's rule, until the An Lushan Rebellion and the decline of central authority in the later half of the dynasty. Like the previous Sui dynasty, the Tang dynasty maintained a civil service system by recruiting scholar-officials through standardized examinations and recommendations to office. This civil order was undermined by the rise of regional military governors known as jiedushi during the 9th century. Chinese culture flourished and further matured during the Tang era; it is considered the greatest age for Chinese poetry. Two of China's most famous poets, Li Bai and Du Fu, belonged to this age, as did many famous painters such as Han Gan, Zhang Xuan, and Zhou Fang. There was a rich variety of historical literature compiled by scholars, as well as encyclopedias and geographical works. The adoption of the title Tängri Qaghan by the Tang Emperor Taizong in addition to his title as emperor was eastern Asia's first "simultaneous kingship". There were many notable innovations during the Tang, including the development of woodblock printing. Buddhism became a major influence in Chinese culture, with native Chinese sects gaining prominence. However, Buddhism would later be persecuted by the state, subsequently declining in influence. Although the dynasty and central government were in decline by the 9th century, art and culture continued to flourish. The weakened central government largely withdrew from managing the economy, though the country's mercantile affairs stayed intact and commercial trade continued to thrive regardless, at least until agrarian rebellions in the latter half of the 9th century brought the dynasty to its knees, resulting in damaging atrocities such as the Guangzhou Massacre.