What happened as a result of the Opium War?
China was opened to Europeans for the first time...ever, basically.
Britain, after subduing much of India, had a lot of resources they could extract from the subcontinent. One of these was opium poppy, used to make opium - an essential ingredient for painkillers. However, when they tried to sell opium in China, the Chinese shut the ports and hired an honest man (important because he didn't accept bribes) to keep them out.
Unwilling to lose one of their best customers so easily, Britain, with some help from France, absolutely crushed China in the First Opium War. The undisciplined horde of Chinese soldiers, some of whom still fought with swords, was no match for the modern British guns.
The effect of this war was that China opened her ports to Europeans to an extent never before seen. Opium wasn't the only commodity that could now be traded in China, either. Europeans set up trading posts all throughout the country, along the coast and rivers, where they could sell all manner of things.
Importantly, though, was the fact that China was now open to Europeans, and not just their goods. Missionaries flooded China, seeking converts after centuries of being locked out. One convert to Protestant Christianity would later establish his own kingdom of followers in southern China, in what would come to be known as the Taiping Rebellion - costing millions of lives.