In the history text book it says, the war was started by the UK, because Qing tried to set a ban on opium trade. Qing government sent Lin Zexu to destroy the opium that remained in the south. UK was mad at Lin so they started the war. UK and France won the war, signed the Treaty of Nanjing with Qing. China was not independent anymore.
But as I see, the war was about trade. Western countries want to trade with China, earn money by selling goods to Chinese, but Qing just didn't want to trade with any country. Selling tea to Western countries made Qing rich. You know, Western countries support free trade, while Qing just want to stop connection with any other countries.
I recommend you to read books about China's Modern Economy.

SORRY, typo in the previous, Elliott is not a Qing official... 

for a long time the British wants to sell their products to China, however, the Qing dynasty(a corrupt governments built by Machu) which then was ruling China was implementing a close-door policy... and was benefitting from a long-time favorable trade balance with the western world, especially their most powerful representative --- the British!

And after the failure of their multi-times attempts in reversing the situation, the despicable British then came up with an evil idea

So, collaborated with the corrupt scumbag officials of the Qing government like Charles Elliott, they started smuggling Opium into China...

a hit to reach the goal of both making money(huge amount of money) and destorying the Chinese as a nation.

some visionary Chinese such as Zexu Lin rose up to fight it, and then as a prelude, there started the history of war for the Chinese to fight for freedom and up against colonialism/invasion which had continued for more than a century...

and ended by the establish of the new China, the People's Republic of China, in the year of 1949.

Thank you Don, I appreciate your response!