As cities began to grow, their influences began overlap, creating arguments between other city-states, especially over land and canals. These arguments were recorded in tablets several hundreds of years before any major war—the first recording of a war occurred around 3200 BC but was not common until about 2500 BC. At this point, warfare was incorporated into the Mesopotamian political system, where a neutral city may act as an arbitrator for the two rival cities. This helped to form unions between cities, leading to regional states.When empires were created, they went to war more with foreign countries. King Sargon, for example, conquered all the cities of Sumer, some cities in Mari, and then went to war with northern Syria. Many Assyrian and Babylonian palace walls were decorated with the pictures of the successful fights and the enemy either desperately escaping or hiding amongst reeds. A king in Sumer, Gilgamesh, was thought to be two-thirds god and only one-third human. His exploits were recorded in many poems and songs of the time.
Of the fortifications we know much more. In the north wall of Nimroud fifty-eight towers have been traced, and at Kouyunjik there are large remains of three walls, the lower part being of stone, and the upper of sun-dried bricks. At Khorsabad there are the remains of a wall, still 40 feet (12 m) high, built of blocks of stone 3 to 4 feet (1.2 m) thick, and the evidences wanting as to finishing of these is completely supplied by the sculptures, which show an extraordinary resemblance to medieval works of the same class. Tier upon tier of walls are represented, enclosing a great tower or keep in the center. The entrances are great arched gateways flanked by square towers. These and the other towers have overhanging parapets just like the mediaeval machicolations, and are finished at top with battlements, remains of which have been found at Nimroud and Kouyunjik, and at Kale Shortage, the supposed capital of Assyria before Nineveh.
Of temples distinct from the palace we have a few supposed remains, but little is absolutely known as to their general form.
But in Chaldea there are some enormous masses of ruins, evidently remains of the vast mounds which formed the substructure of their temples. The grandest of all these and the most interesting is the temple of Nuba at Borsippa (now Birs Nimrod), near Babylon, which has been identified as the temple of the Seven Spheres. This was reconstructed by Nebuchadnezzar, as appears by a well-known inscription. Another example is at Muggier, which was 198 feet (60 m) by 133 feet (41 m) at the base, and is even now 70 feet (21 m) high, and it is clear that both it and the Birs were built with diminishing stages, presenting a series of grand platforms, decreasing in length as they ascended, and leaving a comparatively small one at top for the temple cell. This has been found, it is supposed, at the Birs Nimroud, of vitrified brick made in ancient ovens.
In 536 B.C.E. Babylon fell to the Persian empire. The famed walls of Babylon were indeed impenetrable, with the only way into the city through one of its many gates or through the Euphrates, which ebbed beneath its thick walls. Metal gates at the river's in-flow and out-flow prevented underwater intruders, if one could hold one's breath to reach them. Cyrus (or his generals) devised a plan to use the Euphrates as the mode of entry to the city, ordering large camps of troops at each point and instructed them to wait for the signal. Awaiting an evening of a national feast among Babylonians (generally thought to refer to the feast of Belshazzar mentioned in Daniel V), Cyrus' troops diverted the Euphrates river upstream, causing the Euphrates to drop to about 'mid thigh level on a man' or to dry up altogether. The soldiers marched under the walls through the lowered water. The Persian Army conquered the outlying areas of the city's interior while a majority of Babylonians at the city center were oblivious to the breach. The account was elaborated upon by Herodotus. The Persian invasion began with the fall Opis. The defeat at Opis appears to have ended any serious resistance to the Persian invasion. The Nabonidus Chronicle states that following the battle, "on the fourteenth day [6 October] Sippar was captured without battle. Nabonidus fled." The chronicle's wording implies that Nabonidus was present in Sippar when the Persians arrived. Cyrus remained in Sippar, and then Gubaru, governor of Gutium, and the army of Cyrus without a battle entered Babylon." Nabonidus himself was captured shortly afterward when he returned to Babylon.Babylon