The Opium Wars were two wars between China and Western countries during the Qing dynasty. The first was between Great Britain and China and lasted from 1839 until 1842. The second was from 1856 to 1860 and involved France as well.

It was the British or, rather, the British East India Company, which brought opium to China. They brought it from the Bengal region of India to pay for Chinese goods such as porcelain, 

The wars were about many things other than opium. They were also about opening China to European and American trade and colonizations. Defeat weakened the Qing dynasty.

Opium is a natural substance in the seeds of the opium poppy.  Opium is extracted from the poppy seeds. It was usually smoked for its narcotic effect.  It can be converted into opiates. By 1787, the Company was sending 4,000 boxes of opium (each 77 kg) per year. this trade grew more in the 19th century.

Beginning of the opium trade 
British merchants began selling opium to China.  At that time, opium was grown in India and not in China.  Opium had been used in traditional Chinese medicine for a long time before the British came, mostly to treat disease.   However, opium can also be used as a psychoactive drug that changes the user's state of mind.  Opium is also an addictive drug. 

When the British began importing large amounts of the drug, the Chinese began using opium for its mind-changing effect. More and more people grew addicted to opium. So, the British were able to export more and more opium. By selling this drug, the British slowly began to make more money on their exports to China than they spent on their imports of Chinese goods. British exports of opium to China increased greatly. They went from an estimated 15 tons in 1730, to 75 tons in 1773. Opium was shipped in "chests". Every chest had 67 kilograms (140 pounds) of opium inside.

The First Opium War 
Early in the 19th century, British merchants began to take opium into China in return for bringing Chinese tea back to Britain. In 1839, China said that the British could not bring opium into their country, so the British did not tell China about the opium they had brought with them.  Chinese officials found a lot of opium in Canton, a part of southern China which is now called Guangzhou, and destroyed it. The British were upset at this and sent gunboats in 1840 to attack Chinese cities along the coast. China had no weapons to protect themselves, and so they lost the war.

China was forced to sign the Treaty of Nanking and Treaty of the Bogue. This forced China to open up some of its ports for trade to Western countries, not just Great Britain.  British people in China also received the right of extraterritoriality, meaning when accused of crime they could be tried by their own officials rather than those of China. Finally, China had to give up Hong Kong Island, which became a British colony. Britain had nothing to give back in return, and so these treaties became the first of those known as the "unequal treaties".  Other Western countries soon signed similar treaties with China.

The Second Opium War 
The Second Opium War was also known as the Arrow War or Anglo-French War in China. French soldiers also took a big part in this war, which started when the Chinese took over one of the British ships in the port of Guangzhou, called The Arrow. Britain and France, along with troops from Russia and the United States, attacked more Chinese cities. This included the Battle of Canton of 1857, and in 1860 the Old Summer Palace (Yuanmingyuan) in Beijing was burned to the ground. At the end of this war, the Chinese had to sign more of the "unequal treaties". They included making opium legal in China, opening up all their ports for trade, and to give up part of the Kowloon Peninsula to the British, which became part of the colony of Hong Kong.

In 1898, after the First Sino-Japanese War, Britain would take control of the New Territories under a 99-year lease. At the end of the 99 years, it would return the entire colony of Hong Kong, including the leased land, to China. This happened on July 1, 1997, making Hong Kong the first of two Special Administrative Regions of the People's Republic of China.

References 

19th century in Asia
Wars involving China
Wars involving France
Wars involving the United Kingdom
Wars involving the United States
1850s in France
1830s in the United Kingdom
1840s in the United Kingdom