It is one of the greatest archaeological mysteries of all times: the disappearance of a Persian army of 50,000 men in the Egyptian desert around 524 BC. The Leiden Professor Olaf Kaper unearthed a cover-up affair and solved the riddle.

It must have been a sand storm, writes the Greek historian Herodotus. He tells the story of the Persian King Cambyses, who entered the Egyptian desert near Luxor (then Thebes) with 50,000 men. The troops supposedly never returned; they were swallowed by a sand dune. A fantastic tale that was long the subject of many debates.

Egyptologist Olaf Kaper never believed it: 'Since the 19th century, people have been looking for this army: amateurs, but also professional archaeologists. Some expect to find somewhere under the ground an entire army, fully equipped. However, experience has long shown that you cannot die from a sand storm, let alone have an entire army disappear.'

Petubastis III

Kaper is now putting forward an entirely different explanation. He argues that the army did not disappear, but was defeated. 'My research shows that the army was not simply passing through the desert; its final destination was the Dachla Oasis. This was the location of the troops of the Egyptian rebel leader Petubastis III. He ultimately ambushed the army of Cambyses, and in this way managed from his base in the oasis to reconquer a large part of Egypt, after which he let himself be crowned Pharaoh in the capital, Memphis.'

The fact that the fate of the army of Cambyses remained unclear for such a long time is probably due to the Persian King Darius I, who ended the Egyptian revolt with much bloodshed two years after Cambyses' defeat. Like a true spin doctor, he attributed the shameful defeat of his predecessor to natural elements. Thanks to this effective manipulation, 75 years after the events, all Herodotus could do was take note of the sand storm story.

Kaper made this discovery accidentally; he was not looking for it actively. In collaboration with New York University and the University of Lecce, he was involved for the last ten years in excavations in Amheida, in the Dachla Oasis. Earlier this year, he deciphered the full list of titles of Petubastis III on ancient temple blocks. 'That's when the puzzle pieces fell into place', says the Egyptologist. 'The temple blocks indicate that this must have been a stronghold at the start of the Persian period. Once we combined this with the limited information we had about Petubastis III, the excavation site and the story of Herodotus, we were able to reconstruct what happened.'

The discovery will be announced on Thursday at an international conference. Kaper: 'I expect there to be a great deal of interest in the subject. I look forward to the discussions that will follow.'