Historians have judged the Qing dynasty's vulnerability and weakness to foreign imperialism in the 19th century to be based mainly on its maritime naval weakness while it achieved military success against westerners on land, the historian Edward L. Dreyer said that "China’s nineteenth-century humiliations were strongly related to her weakness and failure at sea. At the start of the Opium War, China had no unified navy and no sense of how vulnerable she was to attack from the sea; British forces sailed and steamed wherever they wanted to go......In the Arrow War (1856–60), the Chinese had no way to prevent the Anglo-French expedition of 1860 from sailing into the Gulf of Zhili and landing as near as possible to Beijing. Meanwhile, new but not exactly modern Chinese armies suppressed the midcentury rebellions, bluffed Russia into a peaceful settlement of disputed frontiers in Central Asia, and defeated the French forces on land in the Sino-French War (1884–85). But the defeat of the fleet, and the resulting threat to steamship traffic to Taiwan, forced China to conclude peace on unfavorable terms."

What was the main problem the Qing had in preventing invasion from Europe?