BERLIN — Not since Harry Truman and Josef Stalin took one another's measure at nearby Potsdam in 1945 has there been such a seismic shift in the balance of power.
At that moment, Nazi Germany and imperial Japan were crumbling. The United States and the Soviet Union were allies-turning-adversaries that were to rule a bipolar Cold War world for half a century.
Now as President Bush and Chancellor Helmut Kohl prepare to meet tomorrow in Washington, they face a world in which the United States and Germany are dominant powers, the first global, the second a European colossus.
When Mr. Kohl was in Washington last spring, Moscow's political structure had not yet fallen apart. The world was recognizable, if just barely.
How the American-German partnership holds together and how a united Germany behaves in its new position is unmistakably crucial to global stability.
Although Messrs. Bush and Kohl constitute a two-man mutual admiration society, it does neither nation any good to pretend differences do not exist.
Germany, in its new assertiveness, bucked White House reluctance at the last Bush-Kohl meeting to insist that Soviet President Mikhail S. Gorbachev be invited to the London Group of Seven summit. He was.
Since then, Mr. Kohl has not been at all reticent in trying to push the United States to extend much more aid to the stricken Soviet peoples -- and soon. He can speak with authority because his country has provided more than $32 billion to Moscow, $17 billion to Eastern Europe and $60 billion to the former East German states -- amounts that make contributions from the United States and other countries a mere pittance.
More provocatively, Germany has shucked some of its postwar inhibitions by siding with Croatia and Slovenia in their attempts to break away from a Yugoslavia dominated by Serbia.
This stand resonates throughout Europe. It causes France, long an ally of Serbia in order to contain Germany, to worry about the old German sphere of influence extending down through the Balkans to the dividing line between Western and Byzantine traditions. Although Bonn has refrained from recognizing Croatia and Serbia, it does so out of professed loyalty to a European Community trying to play peacemaker.
In no way will the German Foreign Ministry confess to a contradiction between its eagerness to sustain some kind of central authority in Moscow and its willingness to see Yugoslavia sundered despite the worries of other EC partners about separatist movements within their own national borders.
President Bush may choose to opt out of the Yugoslav question, to Mr. Kohl's relief. But neither leader can long continue to ignore their stubborn differences on trade.
Last December, negotiations under the General Agreement on Tariffs and Trade (GATT) broke down in part because Mr. Kohl was unwilling to defy his politically powerful farm bloc at the time of a national election.
Bonn took -- and still takes -- refuge in blaming the even more potent French agricultural lobby. But all GATT participants know that only united Germany can break down the protectionist proclivities of the European Community.
GATT issues are often eye-glazing, but if there is one question that really tests the mettle of American-German world leadership, the goal of a more liberal and embracive global trading system is it. Top German officials may feel American complaints are plain wrong, but they are also aware that Mr. Bush's "fast track" negotiating authority may run out if an agreement is not reached this year. It is a litmus test for the quality of Mr. Kohl's statesmanship.
For the United States, Harry Truman's unflinching use of superpower prerogatives now comes naturally despite the pull of America's internal problems. Not so with united Germany as the new European phenomenon, a nation with a gross national product greater than France and Britain combined and the chief banker and bailer-outer of all of Eastern Europe and the Soviet Union.
Chancellor Kohl told a group of American publishers and editors last week that the mere fact that 80 million Germans exist in the heart of Europe is a challenge to their neighbors.
"We Germans recognize the historic situation," he said, referring to the two world wars Germany started in this century.
As a chancellor who already has made his mark as the man who seized the moment to reunify his country, he now sees European unification and a continuing active U.S. presence as his highest objectives.
What seems to worry the German leadership at this juncture is a perceived lack of American understanding of how critical the situation has become in Eastern Europe. The government that seized the moment to unify its country believes the moment must be seized again -- this time to alleviate a frightful situation beyond its eastern border this winter. The specter of mass migration is never far from Germany's preoccupations.
As a German chancellor with emotional as well as intellectual ties to an America that restored his beaten country after Potsdam, that defended Germany against the Soviet menace, that overcame British and French obstructionism to bring about reunification, Mr. Kohl wants the United States to remain in Europe. The obvious question is what he intends to do to bring this about.
A breakthrough on trade policy to prevent the emergence of a Fortress Europe hostile to America would be a suitable response from the dominant economic power in the Euro-Asian continent.
Joseph R. L. Sterne is editor of the editorial pages of The Sun.