When workmen were digging foundations to erect a new Hilton hotel in Mainz, West Germany, they excavated the well-preserved remains of nine Roman warships. Such are the small ironies of history. And now, less than a year later, two more vessels have been uncovered, buried under 12 to 15 feet of clay.
The oldest of the ships was built in 81 A.D., according to the rather precise evidence of the rings in the oak. Most of the ships, however, date from the fourth century, when the empire was far into its famous decline, leading to the sack of Rome by Alaric the Goth in 410. Historians believe the garrison at Maiz, along with this shipyard by the Rhine, must have been abandoned about 10 years earlier.
There is a great pleasure to excavating works of art - the pillar of a temple , the corner of a frieze, a statue. There is a homey satisfaction to excavating domestic remains - the pots and serving dishes of yesterday. The commonality of the day-by-day is a shared comfort across the centuries. Indeed, theories that the primary human impulse is cooperation rather than aggression have been based on the preponderance of cooking utensils over weaponry in recent African diggings.
But what about the souvenirs of war? What about these Roman relics of imperialism - reminders of a territorial imperative over 1,500 years old?
These ancient warships, 30 to 70 feet long, were sleek, purposeful vessels with uncompromisingly straight keels and massive timber frames. There was accommodation for sail amidships, but they were chiefly propelled by oars. In their sharp lines, one feels the thrust of a score of Caesars.
Around 12 B.C., we know, the Emperor Drusus cut a canal from the Rhine to the Zuyder Zee. Some of these ships, part of the classis Germanicus (Rome's German navy), must have traveled on that canal.
How tirelessly the empire laid down arterial roads and bridges and waterways so that its armies could move further, and yet further, from the heart of Rome!
These navies of Rome's many frontiers ferried troops and supplies, patroled against the hostile natives, kept communications open - ruthlessly, making straight lines in a tangled and untidy world. It must have all seemed irresistibly logical to the Romans - the most logical of men.
But in the end, the solution became the problem. One thing led to another - one more bridge, one more canal, one more bronze-beaked ship. There were hardly enough oak trees in the German forests to keep up with the ships. In one 18-year period the Roman navies lost nearly 1,000. There were not enough freed slaves - from Gaul, from Spain, from Africa - to man all those oars. The last words of the Emperor Septimus in 200 A.D. were: ''Pay the soldiers more.'' But there was no longer enough gold to ship out of Rome on those roads and waterways, financing all the garrisons of this garrison state. ''The muscle-bound Marines/ Mutiny for food and pay,'' W. H. Auden wrote in his poem, ''The Fall of Rome,'' adding: ''An unimportant clerk/ Writes I DO NOT LIKE MY WORK/ On a pink official form.''
For what the Romans finally ran out of was will. What was it all for? National security? World order? Manifest destiny? The Romans thought they knew in the beginning.
Toward the end, there was the Emperor Marcus Aurelius, advising: ''Stop being whirled about.'' Don't worry about what other people think, he told himself. Live in the present. Throw away material things. Discover inner peace. What did all that have to do with warships at Mainz - with all the frontier wars that Marcus Aurelius fought as a reflex of Roman duty?
The Roman parallel is always fascinating to Americans. What can we learn from these 11 time-warp souvenirs, raised from the mud like monsters in a horror movie? Some will see them as an argument for more defense; others, as an argument for less defense. Most people will ''learn'' what they are already convinced of.
The ships sit, submerged in huge metal basins in an empty trolley barn, too waterlogged to be withdrawn from water. Polyethylene glycol is being tried as a liquid replacement. But for the moment, air is the enemy. In contrast to their military pretensions, the Roman warships now seem profoundly vulnerable - documentation for a modern historian's conclusion: ''The complete failure of Rome against Germany . . . usefully illustrates the limitations of sea-power.'' And what else? Something in us parallel-seekers wants to know. Something in us doesn't want to know.