What led mesopotamians to move to the river valley?

Gravity mostly. The great weight of the ice moves slowly down the valley.

Though world war 1 and world war 2 have very different conflicts, and circumstances, they also had many simularities. they each fought issues so urgent that every country, felt the need to fight for the side that they had though was right. each country involved had suffered from losses incredibly severe; only in an effort to win.