A letter scratched into a clay tablet reveals a desperate plea for reinforcements that came just too late. Alone, petrified and facing almost certain death, the ancient Assyrian leader Mannu-ki-Libbali scrawled a call for help to his commander, but his cry for extra troops came too late.

Soon after it was sent, the ancient city of Tushan was overrun by Babylonian invaders, its temples and palaces pillaged, then torn down or set aflame.

The letter, scratched into a clay tablet in 630BC, may never have reached its intended recipient. But more than 2,500 years later it has been unearthed almost intact by archaeologists, offering an unprecedented glimpse into the downfall of the one of the most powerful empires of the ancient world.

In the 30-line letter, the author despairs that he lacks the necessary equipment and manpower to stave off the enemy, suggesting that the issue of military resources may be as old as warfare itself.

Archaeologists have identified the author as a city treasurer who had been charged with building up an army to defend Tushan against a coalition of foreign forces, led by the Babylonians.

At its height, in 668-627BC, the Assyrian Empire spanned from Egypt to Iran, and encompassed most of modern Turkey. Tushan, a bustling trade centre and the regional capital, would have been one the empire’s richest cities.

But in its latter years, the empire, mired in corruption and too large to sustain, ultimately fell to an aggressive enemy campaign. The invasion of Tushan is believed to have marked a tipping point in its dissolution.

The letter, found during the excavation of an Assyrian acropolis in southeastern Turkey, gives a remarkable insight into the final collapse of the empire and suggests that the Assyrians may have been militarily unprepared and put up a feeble resistance.

John MacGinnis, an archaeologist from the University of Cambridge who led the excavation, said: “The letter is written during the process of downfall. The chances of finding something like this are unbelievably small.” Mannu-ki-Libbali laments that he has neither the equipment nor the troops needed for the onerous task ahead. He lists cohort commanders, craftsmen, coppersmiths, blacksmiths, bow makers and arrow makers as essential to building a resistance.

It is apparent that all of the above have already fled the city and that he has been left with a near-impossible task. “Nobody mentioned in this letter, not one of them is there!” he writes. “How can I command?”

He also lacked horses, containers, bandage boxes and chariots.

Expecting the imminent arrival of the Babylonians, armed with arrows, spears, boulders and battle rams, the letter ends with the despairing declaration: “Death will come out of it! No one will escape. I am done!”

Irving Finkel, a British Museum specialist in Assyrian history, said that the tablet captured an epic event. “It has almost a Hollywood quality, this sense of the enemy are coming. I can hear their hooves,” he said.

After the invasion, the Assyrian territory was carved up between the Babylonians, and their allies the Medes and the Cimmerians. Half a century later it would be absorbed into the Persian Empire, under Cyrus the Great.

The letter is written on a clay tablet in ancient Assyrian, using a script called cuneiform based purely on lines and triangles. It was written by jabbing a quill with a triangular-shaped nib into wet clay. Different letters were formed by superimposing identical triangles in different combinations.

Dr MacGinnis said: “A complex character might involve around 13 triangles.”

The researchers believe that Mannu-ki-Libbali would have written the letter himself, although there would have been professional scribes at the time. The tablet, which is about the size of a mobile phone, is held by the Diyarbakir Museum in southeastern Turkey, close to where it was discovered.