Using prior knowledge, why did Britain and China go to war over Opium?

I believe it's because China would not trade with Britain and there was a swath of land Britain wanted to obtain from China that they denied them. Afterwards Britain started smuggling Opium into China to get a large portion of their population addicted to create a need for Opium in China. China proceeded to round up every bit of Opium in the country and dump it into the ocean which angered Britain and prompted them to go to war.