Final answer:
The long-term consequence of the historical development discussed in the 1839 letter is the establishment of British spheres of influence in China, weakening China's sovereignty and economy.
Step-by-step explanation:
The long-term consequence of the historical development discussed in the 1839 letter is the establishment of British spheres of influence in China.
When the British Empire militarily coerced the Chinese empire to accept the import of Indian opium, they forced an 'open door' policy on China in which all foreign nations had free and equal access to Chinese ports. This led to other countries, including Japan, Russia, France, and Germany, also carving out their own spheres of influence in China, where they controlled markets through tariffs and transportation.
This development had a lasting impact on China's sovereignty and economy, as it weakened the authority of the ruling Chinese Qing dynasty and led to foreign dominance over trade and resources.