Surely such a weak and unpopular ruler would have made the temptation to invade the Babylonian Kingdom irresistible to Cyrus.
The impending Persian threat of Cyrus forced King Nabonidus to return from Teima around 543 BCE, but it was already too late… by now the people of Babylonia were more interested in Cyrus as their king.
In the month of Tesrit, Cyrus having joined battle with the army of Akkad at Upu on the [bank] of the Tigris, the people of Akkad fell back. He pillaged and massacred the population. The fourteenth, Sippar was taken without struggle. Nabonidus fled. The sixteenth, Governor Ugbaru of Gutium and the army of Cyrus made their entrance into Babylon without fighting. Later, having returned, Nabonidus was taken in Babylon. Until the end of the month, the shield-(carriers) of Gutium encircled the gates of the Esagila …
In the month of Arahsamnu, the third day, Cyrus entered Babylon. (Drinking) straws (?) were filled up before him. Peace reigned in the city; Cyrus decreed peace for all Babylon. He installed Gubaru as governor of (all) the governors in Babylon. From Kislev to Adar, the gods of Akkad that Nabonidus had sent to Babylon returned to their sanctuaries. (Glassner, Mesopotamian Chronicles, p. 237-239)
Cyrus’ first battle against the Babylonians at Upu/Opis was a success, and of great significance. By taking Opis/Upu, Cyrus had flanked the Median wall that stretched all the way to Sippar. This meant that the very next day Cyrus was able to take the city of Sippar without a fight, thus having full control of the Median wall.
This was the wall that was intended to keep out the Cimmerians, Scythians, and any other undesirable barbarians… and it was now in Cyrus’ hands.
When I entered Babylon as a friend and (when) I established the seat of the government in the palace of the ruler under jubilation and rejoicing, Marduk, the great lord, [induced] the magnanimous inhabitants of Babylon [to love me], and I was daily endeavoring to worship him.My numerous troops walked around in Babylon in peace, I did not allow anybody to terrorize (any place) of the [country of Sumer] and Akkad. I strove for peace in Babylon and in all his (other) sacred cities. As to the inhabitants of Babylon [who] against the will of the gods [had/were…, I abolished] the corvee (lit.: yoke) which was against their (social) standing. I brought relief to their dilapidated housing, putting (thus) an end to their (main) complaints.
Marduk, the great lord, was well pleased with my deeds and sent friendly blessings to myself, Cyrus, the king who worships him, to Cambyses, my son, the offspring of [my] loins, as well as to all my troops, and we all [praised] his great [godhead] joyously, standing before him in peace. (Pritchard, The Ancient Near East, Vol 1, p. 207-208)
According to this text, it appears as if the people fully welcomed Cyrus, who only bestowed good to the region. However, this may have been another arrow in Cyrus’ quiver: Propaganda.
“I have not sinned, O Lord of the Lands. I have not destroyed Babylon, nor damaged the Esagila, nor neglected the temple rites.”
Then the high priest of Marduk slapped Cambyses’ cheek! As tears flowed down his face, the god would be pleased and thus concluded the ritual.