In the Early Middle Ages, many empires around the world collapsed into smaller countries. Thanks partly to the Huns invading, the western half of the Roman Empire collapsed into the smaller countries of the Visigoths, the Vandals, the Ostrogoths, the Lombards, the Avars, and the Merovingians. In England, this is the time of the (legendary) King Arthur.
In West Asia, both the Roman Empire and the Parthian Empire fell in the 600s AD to the new Islamic Empire which came out of the Arabian Peninsula. By 800, the Islamic Empire stretched from Afghanistan to Spain. Many of the Christian and Zoroastrian believers who lived in this empire soon converted to Islam. This was pretty much the end of Zoroastrianism as a major faith, but Christianity survived in Europe and much of Africa. Islam also spread across the Sahara Desert to the new African kingdom of Ghana. By the late 700s, Europe was reunited under the emperor Charlemagne, although much weaker than the Islamic Empire.
The Huns also destroyed the Guptan Empire in the 400s AD, and India was also split into many small kingdoms. Indian mathematicians invented the idea of using zero as a placeholder in math about 500 AD, and by 630 the Indian numbers spread to the Islamic empire. But in China, the smaller kingdoms lasted only until 581 AD, when the Sui and then the T’ang Dynasties reunited China. They even took over most of south-east Asia. For the first time, a canal connected the Huai and Yangtze rivers, making it much easier to travel across China.
Even across the Pacific Ocean, there were similar collapses. In North America the Hopewell culture also collapsed about 500 AD into a lot of smaller villages. But in South America, the Maya, the Moche and Zapotec continued to rule.