Opium Wars (from the late 1830s and the 1850s or so)

The British actually declared war on the Chinese for not letting the British East India Company sell opium to their people because the Chinese people were getting addicted to it, which eventually led to two wars fought over this.

Ironically, even France decided to fight alongside the British in the second opium war.

After the opium wars ended, this severely weakened the ruling Qing Dynasty of China and made Britain take a few Chinese ports for trade and acquire control of Hong Kong.