I notice things sometimes.
I'm re-reading Steven Runciman's Sicilian Vespers, determined to read it much more slowly this time, in hope of retaining more. On pp 23f Runciman says that in the mid-13th century, the old Medieval empires were coming to an end: Frederick II gave the Holy Roman Empire a last appearance of splendor; the Papacy was losing its claims to Empire; the Byzantine Empire was in steep decline; the Abassid Caliphate was near its end, while its erstwhile rivals, the Ommayads of Spain and the Fatimid of Egypt, were already gone; and the Sung Empire in China would be gone by 1279. Only the Mongol Empire was still strong, but soon it too would break up into national parts.
Multinational empires were giving way to nation-states. (In the 13th century, "nation" was still very similar to "people who shared one common language.) But what also rose in Europe along with the nation-states, most notably and strongly in England and France but also in Spain and elsewhere?
Mechanical clocks, and guns. No-one knows for sure when they were first made, and there is often controversy over what part of the world can claim to have invented what, but wherever and whenever they were first invented, guns and mechanical clocks were definitely in use in Europe by the early 14th century. By the end of the 14th century there were clock-towers in churches at the centers of towns all over Europe, and guns were known all over Europe and gaining in popularity in European armies.
Is it a coincidence that empires went away at the same time that mechanical clocks and guns appeared?
I have no idea if there's a causal relationship there. It could be a complete coincidence.
A couple of notes: some people say that Ghengis Khan, born around 1162, died in 1227, had huge cannons. This is a linguistic confusion: when guns first appeared, and also for a considerable time afterwards, they were often described with terms which had been used for thousands of years to describe things such as catapults and trebuchets. Also: the Chinese were using gunpowder and rockets for centuries before there were guns in Europe.