BRUSSELS -- Europe's history is littered with noble ideas and broken dreams.

Now there is a new dream: Unity.

Twelve West European nations, among them countries whose differences dragged the world to war twice in this century, have pledged to complete by 1992 what was once dismissed as impossible--an end to frontiers and creation of a single unified marketplace.

If they succeed--and during the past year realists have begun to join dreamers in predicting that they might--the result will reshape the Continent and drastically affect the rest of the world.

U.S. Role Sought

In Washington, the trade aspects of the 1992 reforms are viewed as so pivotal that Secretary of Commerce Robert A. Mosbacher recently shocked Europeans by calling for a direct American role in the process.

"I have advocated a seat at the table at least as an observer," Mosbacher told a Washington audience.

"The European Community's single market may prove to be one of the seminal events of the latter half of the 20th Century," two of the United States' most experienced foreign affairs experts, former Deputy Secretary of State Warren M. Christopher and Deputy Secretary of State-designate Lawrence S. Eagleburger, advised President Bush in a foreign policy analysis.

A unified European Community of Britain, France, West Germany, Italy, Spain, The Netherlands, Belgium, Greece, Ireland, Denmark, Portugal and Luxembourg would constitute the world's largest single consumer market with a population a third larger than that of the United States and a trading volume more than twice that of Japan.

For the United States, the Common Market nations collectively constitute a $1-trillion commercial relationship, by far the single largest American international commitment, dwarfing that with Japan.

Should protectionist pressures turn this fast-emerging internal market into a "Fortress Europe," the consequences would be disastrous both for United States and other global traders.

The new momentum propelling the European Community toward its goals has revived the continent's flagging spirits, dissipating the pessimism that paralyzed it through much of the early 1980s.

Collectively, its leaders have discovered, Europe matters.

As bureaucrats struggle to dismantle myriad barriers by 1992, visionaries talk of grander goals: a common European currency, a popularly elected president and, eventually, full political union.

Youth Enthusiastic

"It's our answer to perestroika and Reaganomics," said Gerhard Fels, head of the Institute for the German Economy in Cologne. "For young people there's an attraction, an enthusiasm about what is happening within the EC."

Community nations have also agreed to draw up a charter of workers' rights and to harmonize health and safety standards. They have earmarked $60 billion by 1993 to cushion the impact of a single market on poor or declining regions.

Vague ideas of European unity floated from the ashes of both world wars, but it was an unassuming French diplomat, Jean Monnet, who gave shape and substance to the present drive.

His vision was a United States of Europe, not of "coalitions between states, but union among people"; and in 1950, Monnet's plan for the war-ravaged French and German coal and steel industries to join in a common market also open to other countries became the precursor to the European Community.