The ancient Greeks called it Pontus Axeinus — the inhospitable sea. Jason and the Argonauts sailed its turbid waters, seeking the Golden Fleece in the land of Colchis, present-day Georgia. Turks who feared its lowering storms called it Kara Dengiz, hence its English name.

Now the Black Sea, contested through history by Roman emperors, Russian czars, Nazi and Soviet totalitarians and, inevitably, by British imperialists in the Crimea in the 1850s, is once again emerging as a strategic amphitheater of clashing interests and aspirations.

When Romania and Bulgaria join the EU in 2007, modern Europe’s new frontier will come hard up against the rumbling underbelly of Russia’s collapsed empire.

Arrayed around this new Black Sea bullring, an encircling host of failed, floundering or would-be states must soon decide whether their future lies within the Euro-Atlantic community.

It is here that defining 21st-century battles over identity, security, democratic values, oil, and migration will be waged. And it is here that an ever-enlarging Europe’s limitations, political as well as geographical, may finally be painfully exposed.

Romania’s reformist leader, Traian Basescu, who watches over a lengthy tract of western Black Sea coast, is keenly attuned to this challenge. His visit to London last week amounted to an early warning. In his view Romania is becoming a frontline state in what governments now call the Greater Black Sea region. “The common security threats that we face are many. The Black Sea region has become an area for trafficking in people, in drugs and weapons,” Basescu said.

“It is an area of frozen conflicts. These are threats for all NATO and EU members. In this region we are in a democratic transition period, a period of emerging democracies — and that presents an element of risk.”

Romania has been offered additional British help in fighting corruption and organized crime, curbing illegal immigration and preparing for EU membership, diplomats said. Both Romania and Bulgaria are already NATO members. And the Bucharest government, which has offered military base facilities to the US at Constanta, has purchased two ex-Royal Navy frigates.

Yet while Romania, Bulgaria, and more precariously, Ukraine, have made their pro-Western choice, the fate of many regional states and peoples hangs in the balance.

Underlying all this is the Black Sea’s growing strategic importance as an outlet for Russian and Caspian oil — another potential source of conflict as well as wealth.

A sort of “best pipeline” contest is now under way. Russia is exploring a new oil route with Bulgaria and Greece that would bypass pro-Western Turkey. From Athens, at least, this looks like a terrific idea.

Another pipeline will run from Azerbaijan via Georgia to Turkey’s Mediterranean coast, deviously circumventing poor, ostracized Armenia. Yet another could link Bulgaria’s Black Sea coast with Macedonia, Albania and the Adriatic — with intriguing implications for the Balkans.

While the EU gazes east and wonders just how far it can go, especially regarding Russia, the US feels fewer constraints. It is determined to secure its Caspian oil supplies. And its new military toeholds on the western shore could in time be used to project US influence across the entire Black Sea region.

Europe’s policy may be drifting. Russia may fret and storm. But Washington reckons it knows which way the wind is blowing. Like the ancient Greeks, it aims to turn the Black Sea into the Pontus Euxinus — the friendly sea.