I am pretty ignorant about population possibilities in Judah and Israel back then. But these numbers intuitively don't seem possible, but like I said this is only a gut feeling.And there was war between Abijah and Jeroboam. Abijah set the battle in order with an army of valiant warriors, four hundred thousand choice men. Jeroboam also drew up in battle formation against him with eight hundred thousand choice men, mighty men of valor.
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Then Abijah and his people struck them with a great slaughter; so five hundred thousand choice men of Israel fell slain.
~~From 2 Chronicles 13.
For comparison sake, in the 1st battle for the Marne in WW1, the total combatants was about 2.5 million men. The US took a total of 263k casualties, 81.7k killed. Germany took 256K total casualties; not sure about how many of these were deaths.
The invasion of Normandy involved 1.3 million combatants and there was a total of about 235k casualties on both sides. obviously the number of dead would be somewhat less than this figure.
Further, these battles were fought with modern warfare not swords and slings and arrows.
Is there the possibility that the huge numbers in Chronicles were copied wrong at some point, perhaps a zero added, for example? I am not opposed to the possibility that 500,000 men died in a single battle on one side, but surely they would have had no army left and I am not even sure if Israel could have mustered a 500,000 man army.
I recognize this has probably been discussed here before; of so I cannot recall it.