system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is ancient Mesopotamia described as a lush land but (the most equivalent) modern Iraq is mostly desert?

I've never been to Iraq and I know that the two regions don't exactly overlap, but still I'm surprised that a country that today is described as mostly desert in the past was the perfect area for agriculture and human wellbeing.
gpt
Civilization formed on riverbeds and river-valleys. Even modern day Iraq has plenty of lush areas fit for agriculture near rivers and sources of water: it's not all desert. 

The cradle of civilization formed between the Tigris and Euphrates Rivers, meaning water was easy to get. The same was happening around the Nile, which floods with incredible regularity causing agriculture planning to be very easy for early civilization.

Additionally climate changes (not *that* climate change) is a thing. The Sahara desert for instance isn't a permanent desert, it has trough the lifetime of the earth flipped between being a desert and being covered in grasses, forests, and lakes as the African humid period went on. It's not impossible to think that the area you're referring to at one point had a very different climate.