The Nabonidus Chronicle, a remarkably well-dated record of the fall of Babylon, helps to establish on what day this event occurred. Following is a translation of a small part of the Nabonidus Chronicle: “In the month of Tashritu [Tishri (September-October)], when Cyrus attacked the army of Akkad in Opis on the Tigris . . . the 14th day, Sippar was seized without battle. Nabonidus fled. The 16th day [October 11, 539 B.C.E., Julian, or October 5, Gregorian] Gobryas (Ugbaru), the governor of Gutium and the army of Cyrus entered Babylon without battle. Afterwards Nabonidus was arrested in Babylon when he returned (there). . . . In the month of Arahshamnu [Marchesvan (October-November)], the 3rd day [October 28, Julian], Cyrus entered Babylon, green twigs were spread in front of him​—the state of ‘Peace’ (sulmu) was imposed upon the city. Cyrus sent greetings to all Babylon. Gobryas, his governor, installed (sub-)governors in Babylon.”g

In any event, secular history definitely establishes that Cyrus was a key figure in the conquest of Babylon and that he thereafter ruled there as king.