How did the geography of the Fertile Crescent help a series of leaders both to conquer and to unify Mesopotamia?

The most salient aspect of the geography of the Fertile
Crescent is the fact that it was a flat plain with rivers running through it.  Both of
these helped to make the area relatively easy to conquer and to
unify.

Areas that are hard to conquer and unify tend to be
divided up geographically.  They may have mountain ranges or jungles that make travel
difficult.  This makes it hard to move armies around to conquer and it makes it
difficult to administer the whole are even if you can conquer it.  The Fertile Crescent
did not have these problems.

Instead, the Fertile Crescent
was easy to access.  The flat plains and the rivers made travel relatively easy.  For
this reason, armies were able to come into the area easily and conquer.  Once this was
done it was not too hard for them to unify the area because they could move around it
easily.