The correct answer is - a-desert.
The Fertile Crescent was a place of significant development of civilizations and it was a place were farming was on an another level, and was due to the Tigris and Euphrates Rivers, but everything around the Fertile Crescent was desert, on all four sides. The desertous landscape was a big obstacle for any foreign force to be able to successfully launch a military campaign on the civilizations in here. In the period when the Fertile Crescent was one of the most prosperous regions in the world the militaries were still pretty immobile and slow which is equal to suicide if they tried to pass deserts and than go into war.
