Rome had the reputation of being unbeatable. It held it for one hundred and fifty years. By the mid-2nd century AD, though, it was showing signs of some wear and tear. Emperor Marcus Aurelius spent most of his time defending the vast empire. Rome had gotten too large. It was starting to fall apart. After Marcus Aurelius died, his son spent huge amounts of money on entertainment. He was killed, and so was his successor.
The job of emperor was sold to the highest bidder, but the winner was quickly replaced by Septimius Severus, who was a good ruler for 14 years. He was followed by other members of his family, who were all killed. After that followed a time that was hard for everyone. Taxes were high and many people became outlaws to avoid taxes. This time was called the "Anarchy".
Finally, there came a ruler who realized he couldn't control Rome, the Emperor Diocletian. He split it in half and gave the other half to a general named Maximian. Diocletian did a good job ruling, although he did work a little too hard to restore the status of emperor. He declared himself a god and wore a pearl crown, and made all visitors kneel and kiss his robe.
Then began the religion of Christianity. A Jewish carpenter named Jesus taught that people should give up their sinful ways and devote their lives to serving God and other people. He was, sadly, crucified by the Romans. The Jew were the people more responsible for that, actually, but that's a whole other story, which I will probably cover some other time. The Romans didn't like this new religion, which did not allow people to bow down to other gods, which meant they would not worship the state gods or the emperor. They were punished and often underwent horrific tortures.
But they would not give in. Christians worshiped in secret frequently.
After many years of rather insane rulers and rulers who hated Christians, Constantine became emperor of Rome. Once, before going into battle, he saw a cross in the sky. He believed it was a sign from God. He allowed the Christians to worship freely and was baptized on his deathbed.
Then came a time when barbarians began to invade. They aided Rome's fall like carbonation aids water in tasting like pop. Attila the Hun was one of the leaders of these tribes. He, as a young man, was exchanged with a young Roman man as a hostage. If the Huns attacked Rome, Rome would kill the hostage, and if Rome attacked the Huns, the Huns would kill their hostage. Attila spent his years in Rome learning strategies. He used this knowledge in his later campaigns. He was (and still is) known as a barbarian, who was merciless. But there are a few things he did that are not well known. He is considered a good man in Romania or Germany or somewhere in that area, because he was a sort of founder for a country somewhere around there. He was also known (then, anyway) for his generosity toward conquered cities (if they gave up willingly, of course). He also usually had a pretext for whatever he was doing. Once, a member of the emperor's family asked to marry him.(They must have suffered from bouts of insanity. No one with a brain would do that without a really good reason.) But her family found out and refused to let her. Attila used that as an excuse to attack Rome. Clever.
But Attila the Hun isn't the only one to blame for Rome's fall. There were other barbarians, too. And there were emperors who were so corrupt they were to blame partially. But no one has ever figured out a for sure reason why Rome collapsed.