WHEN the Berlin Wall fell in the fall of 1989, a number of critics complained that President Bush's response was too tepid - Senate majority leader George Mitchell even suggested rather absurdly that the leader of the free world should fly to the Wall to emote about the end of the cold war in a speech. Such an act would have been seen in many capitals as the worst kind of US grandstanding.Yet Mr. Bush could have been more of a cheerleader in '89. More than the $200 million the White House offered for East Europe was probably in order. Now change of an even greater order of historic magnitude is at hand - the dissolution of the Soviet Union, the crumbling of a regime that has captured American fears, rhetoric, strategy, and wallets for nearly 50 years. Bush handled the coup with aplomb. But soon the US president must respond both in words and initiatives to the new opportunities the Soviet breakup presents. In this regard, Bush may be too much the "prudent" statesman. One hoped for more than his statement from the golf course last weekend on the collapse of the Soviet Communist Party, which Bush underwhelmingly called "another welcome step in the reform process." The White House strategy has been for the Soviets to show they are worthy of aid by making clear-cut reforms. The burden has been on Mikhail Gorbachev to show he is serious - a burden made difficult by hard-liners. Now, with hard-liners in retreat and with power shifting to Boris Yeltsin and the republics, the entire dynamic has changed. British Prime Minister John Major and German Chancellor Helmut Kohl, on opposite ends of the aid question during the G-7 conference in London (Mr. Kohl favored aid), are now talking collegially about what Europe can do to assist the Soviets. Rather quickly, the burden may shift to Bush to explain why the US doesn't have a serious aid strategy but is making vague calls for further Soviet reforms. In four years the Soviet Union has opened up freedom of worship, thought, and expression; an arms control treaty is ready; Soviet troops are pulling out of East Europe; republics are holding local elections and negotiating trade agreements; Moscow is poised to grant the Baltics sovereignty; the party and hard-liners are on the ropes; and this winter food may run out. Discussions about a mini-Marshall Plan of technical aid followed by direct assistance should begin. The West has remarkable leverage. Aid would be tied, for example, to the shaping of a new union treaty, the recognition of the Baltics, the end of aid to Cuba, and graduated market reforms, starting with privatization. Here's hoping the administration is only waiting for the right time to come out with the news. Two years after the revolutions in Eastern Europe, a chief concern among some of the savvier reformers there is of a subtle reemergence of the old (and well financed) communist parties - operating under different names, of course. Should some of the extravagant capitalist promises of certain East European politicians not pan out, where will people turn? Now is the time to block any regression - both in Eastern Europe and the Soviet Union.