The opium war was significant for britain because it led to

Total control of the Opium trade in China by the United Kingdom and the annexation of Hong Kong.

Explanation:

The Opium Wars were two warlike conflicts that occurred in the 19th century between the Chinese and British empires. The First lasted between 1839 and 1842. The Second, in which France became involved with the British, exploded in 1856 and lasted until 1860. Its causes were the commercial interests that created British opium smuggling in India and China and the efforts of the Chinese government to impose its laws on that trade.

In the spring of 1830, faced with the alarming and unrestrained abuse of the opium trade committed by the British in China, Emperor Daoguang ordered Lin Hse Tsu to combat this plague quickly, and he responded by stopping the corruption of the imperial civil servant and ordering the destruction of more than 20,000 Opium boxes. Lin Hse Tsu sent a letter to Queen Victoria asking her to respect the rules of international trade and not trade with toxic substances.

The British responded attacking China. The Chinese defeat in the two wars forced the government to tolerate the opium trade. The British coerced the Chinese to sign the Unequal Treaties, opening several ports to foreign trade and annexing Hong Kong.