What went on in 400ad?

Answer 1

In Europe, the Roman Empire was on the decline, succumbing to increasing invasions from forces from the north and east. The Empire had already become Christianized, and the city of Constantinople was becoming more influential than Rome as a culture center. The city of Alexandria, Egypt had been devastated by a powerful earthquake and tsunami some decades before. In the British Isles, the Roman presence had sharply diminished, returning those lands to the tribes of Anglo-Saxons which the Romans had previously subjugated. Soon afterward, most of Europe would languish for centuries in the "Dark Ages," a time of stagnant development.

In the Americas, unknown to Europeans, the Inca, Aztec, Olmec, and Maya cultures were flourishing in sharp contrast. Cities like Chichen Itza, Uxmal, and Dzibilchaltun in what is now Mexico rivaled some of Europe's architecture.