China was the leading power on Earth, but within a century, the momentum of history shifted to Europe.
In 1400, in many ways, China was the leading power on Earth. As it is today, China was a huge country geographically, with a huge population. Chinese history reached back three thousand years and more; Chinese technology was the best in the world overall, and nowhere was that more apparent than in their maritime technology. Ocean-going Chinese junks of that day stacked up well against any wooden sailing ship until the nineteenth century. They were huge, built with watertight compartments; Chinese navigators had the scientific knowledge and the technology to sail in blue water, out of sight of coastlines. They regularly sailed to East Africa, and may have circumnavigated the globe.
China was ruled by a monarch with absolute powers, which could be a strength or a weakness, depending on who occupied the imperial throne. Chinese history pivoted when a new emperor took power early in the fifteenth century. He was an inward-looking man, uninterested in anything outside China. He ordered all the ocean-going junks burned, along with all accounts of their voyages. We know something of that era in Chinese history largely because someone in the legendary Chinese bureaucracy disobeyed the emperor, and kept some accounts.
On the other side of the Eurasian landmass, around 1450, tiny Portugal began an unlikely role. It was a nation of roughly a million people, most of whom farrmed the poor, rocky soil. Portugal faced the Atlantic, however, and its royal family looked outward for the future. The king and his brother undertook a remarkable program of systematic exploration. Over roughly the last half of the fifteenth century, Portugal sent ships down the west coast of Africa, gaining knowledge, building bases, and pushing ever farther south. Their goal was to find a sea route around Africa to the markets of India, to take trade away from the overland caravan route.
They evemtually succeeded, but the process of exploration proved far more important. Portuguese sea captains developed a thorough knowledge of the wind and sea currents of the east central Atlantic. Columbus used that knowledge to sail west. The Portuguese bases down the African coast put them in contact with the natives. That led to some trade. Unfortunately, it also led to the beginning of the African slave trade.
With the slave trade, Columbus' discoveries, and the emphasis on overseas expeditions as a way to increase wealth, the European dominance of the modern world can be traced back to an aggressive, determined Portugal. Not quite two centuries after the Black Death had taken perhaps a quarter of Europe's population, Spain had a huge, rich empire, Portugal had its own empire, and much of the rest of Europe was scrambling to catch up.
The next time the Chinese saw sailing ships on par with their fifteenth century junks, the ships belonged to the British Royal Navy. The British ships were in Chinese waters, and the encounter did not go well for China.