Final answer:
The cause of the Opium War was the conflict between the Chinese government's attempt to ban the opium trade and the British desire to continue selling it. The war resulted in the Treaty of Nanjing which forced China to surrender Hong Kong to Britain and open ports for European traders.
Step-by-step explanation:
The cause of the Opium War was the conflict between the Chinese government's attempt to ban the opium trade and the British desire to continue selling it. After the Chinese government seized opium stored in British warehouses, the British sent military forces to China in the First Opium War. The British used their superior military technology to defeat the outdated Chinese military, resulting in the Treaty of Nanjing which forced China to surrender Hong Kong to Britain and open ports for European traders.