The British fought the first and second opium wars on a half-hearted basis. Part of that was moral repulsion on the parts of a minority of British politicians. Part of it was the irresolution of the British commanding officer, Lord Elgin, who thought the whole war immoral. The British basically fought when they finally got tired of Chinese delaying actions and measures that basically painted the British as barbarians. The British found the Chinese sense of superiority infuriating, particularly given the massive gap between the two nations in military firepower, China was back in the medieval age while the British had the most formidable weapons in the world.
Let’s suppose that the British sent another commander instead of Elgin. Someone like Clive or Rhodes, who will see the opportunity presented by the war for British expansion and the courage to defy the government. The Chinese viceroy makes his provocations as in OTL and our commander swings into action. There is no question that the British could have taken Canton with the forces available. The Chinese people hated their viceroy and wanted to return to business as usual. The Chinese army was demoralised, particularly after the first clashes. The British effectively destroy the Chinese forces in the area and occupy Canton. The commander annexes the city and the surrounding regions to Britain.
The Chinese emperor appears to have never been told how bad the defeats were in OTL until the British/French force reached Peking in the second war. The odds are that the Chinese government will continue its arrogant actions, which will convince the British (as was considered in OTL) that the government needs changing.
The British proceed slowly, as they did in India, raising some native forces and using them as scouts. The British also destroy the restrictive land rules that made the lives of the peasants a misery. This makes them more popular and pro-British peasant revolts break out across china. This movement co-opts the semi-Christian tai-ping rebellion that took place after the first war and transforms it into a pro-British movement. Our Commander takes advantage of this to start seizing the other coastal cities, before marching on Peking.
The Chinese will probably have grasped the disaster by now. They’ll try to negocaite, but the British will have run out of patience with Chinese evocatism and will demand a complete surrender. The British will probably follow the Indian precedent and offer to allow the Chinese emperor and his court exile somewhere luxusious in exchange for a peaceful transfer of power. The British formally announce their annexation of China and start running the place, starting with the coastal cities.
Assuming the British follow the Indian precedent again, they’ll probably offer the mandarins of western China limited independence, provided they toe the British line and pay homage to queen Victoria. The areas of china that are directly ruled by the British will probably improve quickly, the British won’t exploit the peasants as ruthlessly as the Manchu’s did and they can introduce new techniques. The Chinese would also have lost the deadening hand of the imperial broughcraticy.
The Japanese would be taken far less seriously if they have nowhere to conquer. The British or Americans might take over there as well, while the French and Germans might put more effort into colonising Africa. Russia and Britain will have a long cold war if they have a longer land border and the British would probably react by introducing weapon factories, training and then deploy a few million Chinese troops to the borders. The Russians would probably annex Manchuria as the rest of China fell.
