Britain declared war.

Explanation:
When Lin Ze-xu announced that Canton was closed to foreign trade in 1839, Britain reacted by declaring war on China. This event is known as the First Opium War. The British were outraged by the closure of Canton, as it was a major port for British trade in China, particularly in opium.

- Background on the Opium Trade:
The British had been involved in the illegal opium trade with China for several decades. Opium, produced in British-controlled India, was smuggled into China and sold to Chinese merchants. This trade was highly profitable for the British, but it had devastating effects on the Chinese population, leading to addiction and social problems.

- Lin Ze-xu's Efforts:
Lin Ze-xu, a Chinese official appointed to deal with the opium problem, took drastic measures to combat the trade. He ordered the confiscation and destruction of large quantities of opium and closed the port of Canton to foreign trade. Lin's actions were seen as a direct challenge to British interests in China.

- Britain's Response:
The British government, under pressure from British merchants and traders, demanded compensation for the confiscated opium and the reopening of trade. When their demands were not met, they sent a military expedition to China in 1839. The British navy launched attacks on Chinese coastal cities and ports, including Canton, to force the Chinese to reopen trade.

- Outcome:
The First Opium War lasted until 1842 when the Treaty of Nanking was signed. Under the terms of the treaty, China was forced to pay a large indemnity to Britain, cede the island of Hong Kong, and open several ports to British trade. This marked the beginning of a series of unequal treaties between China and Western powers, which weakened China's sovereignty and control over its own trade.

- Significance:
The First Opium War had a significant impact on China's relationship with Western powers and the trajectory of its modern history. It exposed China's military weakness and led to further encroachments by Western powers, contributing to the decline of the Qing dynasty and the eventual fall of the imperial system in China.