ROME, DEC. 15 -- Twelve European governments took a historic step toward political and economic union today by launching plans to surrender some of the most central powers of the nation-state: control over money and the making of foreign and defense policies.

Leaders of the 12 European Community countries concluded a two-day summit meeting, and the foreign and finance ministers began a pair of conferences aimed at establishing a single currency in this decade and merge other policies so that Europe can assume responsibility for its own security and speak with one voice to the world community.

The ambitious drive toward a virtual United States of Europe has gained momentum in recent months from the rapid fusion of the two Germanys and Iraq's invasion of Kuwait, which has reminded many Europeans about their vulnerability to distant crises that can jeopardize their common security.

The twin set of intensive negotiations are expected to produce amendments to the EC's founding Treaty of Rome that will allow majority voting on issues of fundamental national importance, so that no single country can block the most difficult decisions.

The talks are expected to finish in about a year, so that the results can be ratified by each nation's parliament and take effect by the end of 1992, when the EC will eliminate the last barriers to the free circulation of goods, capital and people in an internal market of 340 million citizens.

Italian Prime Minister Giulio Andreotti said the "unreserved approval" by the 12 leaders of the two inter-governmental conferences was "better than what one could have imagined some time ago or even a couple of days ago."

Andreotti, who presided over the summit, hailed "the climate of great cooperation animated by a desire to reach agreement." Italian officials said the remark was a clear reference to the conciliatory spirit toward the European process shown by Britain under its new prime minister, John Major.

The last European summit here at the end of October ended with Britain isolated from the 11 other members by then-Prime Minister Margaret Thatcher's staunch opposition to a single European currency and to further relinquishing national sovereignty over foreign policy and defense. Thatcher's stubborn stand evoked criticism from key members of her cabinet and set in motion a challenge to her Conservative Party leadership that led to her resignation.

Major described the summit outcome today as "a great victory" and likened the forthcoming discussions on European political and economic union to a great banquet "with a menu of proposals that contains some of our favorite dishes." But he foreshadowed some difficult bargaining by noting that "the Community as a whole has not yet ordered what will be served."

Despite the harmonious tone that prevailed at the summit, several leaders implied that Britain still looms as a roadblock to those who seek dramatic progress toward abolition of national political and economic structures. Asked his opinion about Major's stance, French President Francois Mitterrand said curtly that "there has been a change of personalities but not of policies."

Britain has promised to produce its own proposals in the coming weeks for an alternative European currency, known as the "hard Ecu," that could function along with the 12 national monies. But other governments, as well as EC Commision President Jacques Delors, insist that the debate on economic union is closed and a timetable has been set.

Delors, who says he is "deeply suspicious" of British intentions about the alternative currency, warned that if certain countries, presumably Britain, seek to reopen the debate, "it will provoke a political crisis."

The European leaders agreed at their last summit to a three-stage process that will create a European Central Bank in 1994 and gradually invest it with the necessary financial powers to govern a single European currency by the end of the decade. Thatcher vehemently opposed this plan. While Major is considered one of the authors of the British hard Ecu plan, officials close to him say he has not ruled out accepting the idea of a single currency.

The most difficult task facing the negotiators will be to flesh out the fuzzy vision of European political unity. In their final declaration, the leaders set forth guidelines, but in several cases they contain inherent conflicts that will require major concessions by several governments.

The European governments agreed that the "democratic legitimacy" of their political union must be enhanced by stronger powers for the European Parliament, which will require national legislatures to cede more of their control -- an idea that arouses resentment and suspicion in Britain because its own Parliament has such a long and proud history.

Germany, whose 16-state federal system makes it one of the most decentralized countries in Europe, is an ardent advocate of a powerful European Parliament. But France, perhaps the most centralized nation in Western Europe with virtually all key decisions made in Paris, insists that national leaders should make all major decisions for the European Community at their summits.

The institutional conflict, pitting the European Parliament and the EC's Executive Commission against some elected national leaders reluctant to cede power, could create unusual alliances. Britain and France appear aligned against Germany, Italy and the smaller European nations that want European bodies to gain more powers.

But on foreign and security issues, France and Germany have formed a powerful coalition. Before the summit, Mitterrand and German Chancellor Helmut Kohl produced a joint initiative calling on European countries to seize more control over their collective security by exploring ideas like an integrated European military command.

One way to unify military control, which the leaders want discussed in the coming talks on political union, would be to fold the nine-nation Western European Union (WEU) into the European Community. The WEU, a moribund institution but the only existing European defense organization, consists of all EC members except Denmark, Greece and neutral Ireland.

But Britain, while willing to explore a larger role for the WEU, does not want to jeopardize the future of the North Atlantic Treaty Organization or encourage the departure of American troops from Europe and the erosion of U.S. military leadership of the Atlantic alliance.

To assuage those concerns, the European leaders declared today that while a future political union should extend to security and defense policies, it should do so "without prejudice to current obligations of member states and take into account the importance of maintaining and reinforcing existing links within the framework of the Atlantic alliance."

Nonetheless, a more unified Europe is showing greater independence from the United States, especially in areas where they compete.

The European Community's refusal to accept U.S. demands to reduce agricultural subsidies led recently to the collapse of global trade talks. While the government leaders took no new initiative to break the deadlock, the EC's Delors harshly denounced the United States "for trying to isolate us in the world, because they won't succeed."

The Americans share responsibility for the failure of the trade talks because, Delors said, they "are asking too much, too soon, and they should not tell us how to organize our rural farming life."