According to this study, what was Belshazzar's fatal mistake as he enjoyed the party the night Cyrus conquered Babylon? 522 - 5-4

Pride led Belshazzar to make a grave military mistake by being complacent due to the seemingly impregnability of the city. Rather than being on the alert the arrogant king kicked back with his subjects and partied. Daniel 5:1, "Belshazzar the king made a great feast to a thousand of his lords, and drank wine before the thousand." Meanwhile the River Euphrates that protected the city was diverted allowing the Persians to cross and enter the city.

As Belshazzer and his guest were drinking, in his arrogance he committed a fatal mistake; Daniel 5:2, "Belshazzar, whiles he tasted the wine, commanded to bring the golden and silver vessels which his father Nebuchadnezzar had taken out of the temple which was in Jerusalem; that the king, and his princes, his wives, and his concubines, might drink therein.

As an even further insult, Daniel 5:4, "They drank wine, and praised the gods of gold, and of silver, of brass, of iron, of wood, and of stone." It is hard to comprehend the arrogance of Belshazzar in using sacred vessels stolen from the Jewish Temple to drink from and toast his pagan gods. However, it is easy to understand the anger of our righteous God.

Suddenly the “Hand writing on the wall” appeared, “Mene, Mene, Tekel, Upharsin.” So much for pride and spitting in the eye of God. In keeping with prophecy, Belshazzar embarrassed himself in public.

Mene meant that God had numbered the kingdom of Belshazzar and finished it. Belshazzar was weighed in the balances and found wanting. His kingdom was divided, and given to the Medes and Persians. Daniel 5:30, “In that night was Belshazzar the king of the Chaldeans slain.”