It should be noted, however, that these boundaries were quite fluid, Assyria spreading S of the Little Zab when Babylon weakened, but retreating when Assyrian political fortunes were low and those of Babylon were in ascendancy. Such fluctuation was true of the other boundaries and particularly that of the Tigris, as Assyria early extended its influence W of that river. The Assyrian Empire, of course, came to embrace a far larger area.​—MAP, Vol. 1, p. 954.

There was a continued close relationship between Assyria and Babylon throughout their history. They were neighboring states jointly occupying a region with no real natural division to serve as a frontier between their territories. The region of Assyria proper, however, was mostly a highlands area, generally of rugged terrain, with a more invigorating climate than that of Babylonia. The people were more energetic and aggressive than the Babylonians. They are represented in carved reliefs as of strong physique, dark-complexioned, with heavy eyebrows and beard, and prominent nose.

Assyria’s religion was largely inherited from Babylon, and although their own national god Asshur was viewed as supreme by the Assyrians, Babylon continued to be viewed by them as the chief religious center.