They pissed off the British twice at the height of their power, when they had steel ships and China only had wooden skiffs.

The first time the destroyed opium which pissed them off because they destroyed product without compensation and got their ass kicked by the British. This is how they got Hong Kong

Second Opium war, they impounded a ship registered to Hong Kong (British) so a combined British/French force proceeded to kick their ass till they sued for peace, they then tortured and even killed the diplomats sent that ended up pissing off a British commander that was against the war, he led a force straight to Beijing, found the remaining diplomats, looted then burned down their Palace.

This all began because China thought it was superior to everyone and can do what it wanted, it was self inflicted decline like believing you can bully the kid that is 6ft and takes boxing lessons.