The british had a trade industry with china, but trade regulations were strict there.
A Diplomat was sent to neogotiate better trade deal.
Sadly the envoy was turned away with little result.
To make matters worse, 2 drunk british sailors killed a chinese man, but were tried by british law, rather than chinese law, which the chinese were quite upset about.
It seemed war was inevitable, and eventually the british sent an expeditionary force to china, which captured the capital. then the british got what they wanted and then some.
To this day, the events of this war are a benchmark for how Chinese to western politics are going, these events influenced their dealings with foreign countries to this day. The war also shattered the monarchy of china, and made it a republic in the end.