The dividing line between ancient and modern Chinese history is?

The Opium War of 1840

The defeat of the Opium War and the signing of a series of unequal treaties led to a fundamental change in the nature of Chinese society. Foreign capitalism got from China a series of privileges such as the cession of Hong Kong Island, the compensation of 21 million taels of silver dollars, the opening of the five ports of Guangzhou, Fuzhou, Xiamen, Ningbo, and Shanghai for trade, as well as the right to agree on tariffs, the right to consular adjudication, and the one-sided most-favored-nation treatment, which seriously undermined China's independent sovereignty.

After the signing of the Treaty of Nanking, the United States and France took advantage of the opportunity to claim privileges and forced the Qing government to sign a series of unequal treaties. The Opium War marked the beginning of China's modern history, and from then on, China began to experience deeper suffering, and the Chinese people faced a more complex and tortuous struggle.

Expanded Information:

The serious harm that the opium trade had brought to the Chinese society aroused the attention of the Qing government and the people at large. The Qing government enacted the ban eight times from 1821-1834 in its own interest; a part of the ruling class, witnessing the social crisis, demanded reform of the maladministration and a strict ban on opium in China.In December 1838, Emperor Daoguang ordered Lin Zexu to be the minister of imperial investigation and sent to Guangdong to prohibit the smoking of opium.?

When Lin arrived in Guangzhou in March 1839, he ordered foreign traffickers to hand over all opium and promised not to sell it again, promising that "subsequent ships would never dare to carry opium, and that from June 3 to June 25 most of the opium stocks legally belonging to the British would be destroyed. The British viewed the Chinese anti-smoking campaign as a violation of private property and found it intolerable, leading to the outbreak of war.