What do you think was the major reason for the decline of ancient societies?

Many reasons, conquests by other cultures, diseases, moral decay, natural disasters

There are a lot of reasons for the decline of ancient societies. Some, like the Minoan civilization, were destroyed because of natural calamities. Most were pillaged and brought down to rubbles because of wars and raids. Others, like the Babylonian civilization, declined because of the introduction of new cultures, customs and traditions because of immigrants. There are also societies, like the Native Americans, who were virtually destroyed because of the greed of advanced cultures - driven away from their homes and stripped of their land.