The North Atlantic Treaty Organization agreed tonight to deploy U.S. medium-range nuclear missiles in Western Europe, a momentous decision in alliance history that was marred by the unwillingness of the Netherlands and, to a lesser degree, Belgium, to endorse the plan fully.

Secretary of State Cyrus Vance and Defense Secretary Harold Brown sought to put the best possible light on the NATO decision, calling it "unanimous." But the reservations expressed by the two countries reflect a setback for American hopes of a more complete display of alliance unity on such a crucial issue and one strongly opposed by the Soviet Union.

The range of the new weapon means that, for the first time, U.S. ground-launched missiles will be able to strike deep inside the Soviet Union from Western European bases. The weapons are meant to balance some 120 new Soviet 2,500-mile-range SS20 mobile misiles already deployed plus scores of new Soviet Backfire bombers.

Their basing in Western Europe, however, has made these weapons extremely sensitive politically in the NATO alliance. The Soviet Union bitterly opposes their deployment and unleashed a heavy campaign of conciliatory gestures and threats to try to pressure the European allies not to go along.

The final NATO agreement, reached after seven hours of tense debate behind closed doors, means that the United States, with congressional approval, will soon begin production of 1,000-mile-range Tomahawk cruise missiles. Installation is to begin by late 1983 at U.S. bases in Britain, West Germany and Italy.

All NATO countries will pay part of the costs, but the United States will pick up the entire estimated $5 billion of development and production costs. The European share of operational costs is expected to be about $500 million.

The plan calls for all 108 Pershing and 96 cruise missiles to be based in West Germany. Britain will take 160 cruise missiles and 112 will go to Italy. Of the planned total of 572 missles, the Netherlands and Belgium each were supposed to take 48 cruise missiles.

The Dutch government, however, in a separate statement issued here tonight said it will not make a decision to allow that deployment until December 1981. It will then decide on the basis of whether arms control negotiations with the Soviets have achieved any concrete results.

In combination with the arms modernization plan, NATO also proposed beginning new arms control negotiations with the Soviet Union "as soon as possible" to try to limit the number of these new weapons on both sides.

The proposal says any reductions "must be consistent with the principle of equality between the sides."

The proposal also calls for a unilateral Western withdrawal of 1,000 older U.S. atomic weapons in Europe as a response to Soviet President Leonid Brezhnev's recent gestures to withdraw some troops and tanks from East Germany.

Dutch Foreign Minister Chris Van der Klaauw said the decision was a very difficult one for his country. The Netherlands "fully agrees that an effective answer must be found for the increasing Soviet military threat," he said, but the Dutch believe the first Western priority should be negotiations with Moscow. The question in the Netherlands, he said, is how to stop the arms race.

Because it will take some three years to build the new weapons, the Dutch theoretically could decide in two years to join the plan.

The Belgian position is much more positive than that of the Netherlands, said NATO Secretary-General Joseph Luns. The Brussels government has basically accepted the NATO production and deployment, he said, but has reserved the right to review that decision six months from now, also on basis of whether progress has been made in arms control negotiations.

Since very little is likely to happen in reaching any arms agreement with Moscow in such a short time, the Belgian position is seen here as basically a political tactic meant to appease elements within Belgium's five-party coalition government.

Considering the political problems of some governments, Vance said the decision was a "very successful one" for the alliance in responding to the Soviets.

Nevertheless, the agreement has a degree of fragility in the sense that for a while at least, there is very little cushion for NATO to fall back on. f

The coalition governments in the Netherlands and Belgium are both shaky. If a political crisis developed in Italy, that would leave West Germany as the only nonnuclear continental nation to provide a home for these weapons. West Germany, along with Britain and Italy, has given strong support to the program, but a change of conditions could create political problems in Bonn.

The United States has had about 7,000 atomic warheads in Europe for years, but virtually all are short-range. The 572 missles would be included in the new level of 6,000 weapons.

A special new NATO consultative group will be set up to support the United States in negotiations, and Vance speculated that preliminary discussions might begin in the next month or two.

The Soviets had sharply attacked the prospective NATO decision, but recently indicated that a decision merely to go ahead would not necessarily wreck changes for negotiations in the intervening years.

Vance's description of the NATO desicion as "unanimous" apparently referred to the fact that the 14 nations represented at today's meeting went along with the agreement and the inclusion of the two reservations.

Vance said the best place for the negotiations would be the next round of U.S.-Soviet strategic arms limitation talks and hoped the current SALT II treaty now before the Senate would be passed soon. Luns, however, said SALT was not the only possible forum for new talks.

In response to questioning, Vance rejected the idea that the Soviets will interpret today's decision as a lack of NATO unity that they might be able to exploit by continued heavy pressure on the wavering countries.

Just a few hours before the final decision, U.S. officials told reporters privately the United States was holding out for a firm declaration without reservations. This could have meant that the Neatherlands might have been isolated competely.

The problem, U.S. sources said, was that if the Netherlands was allowed a formal reservation, then Belgian domestic politics would require that the Belgian government also be allowed some flexibility.

In the end, that is what happened despite the earlier U.S. hopes for a more firm covering up of the differences.

Dutch officials stressed tonight that their decision was linked only to the nuclear issue and that the Netherlands remains "faithful to the alliance."

The missile decision is so important to the Carter administration's strategic planning that the need to concentrate on it today momentarily swept aside high-level administration concern with the Iranian crisis. Thursday, however, NATO foreign ministers are expected to take up the problem of the U.S.-Iranian confrontation.

The planning to modernize NATO's nuclear arsenal started two years ago and West German Foreign Minister Hans-Dietrich Genscher said today that no previous NATO decision had ever been so well prepared as the one made today.

The United States and its allies were determined to produce as united a front as possible to avoid the kind of political strains between Washington and Western Europe that erupted in 1977 when President Carter abruptly decided against producing the neutron bomb.

In many ways, today's decision is the most important in NATO's 30-year history, with the possible exception of the formation of the postwar West German army.

It is the first time that the Europeans have been so directly confronted with the question of their own involvement in nuclear warfare. While thousands of U.S. nuclear bombs, shells, mines and short-range missiles have been based in Europe for years, they have not been able to reach the Soviet heartland.

Even though the weapons are under U.S. control, their deployment will assure that Western Europe is etched deeper into Soviet targeting plans.

The new weapons also have opened up the arcane yet crucial questions of "coupling." Traditionally, the United States has said that a nuclear attack on its allies would be met by a nuclear response from the United States.

But now, critics of the decision argue, it is conceivable that a Soviet nuclear strike at western Europe might be met by a U.S. counter-strike from Europe, thus possibly "uncoupling" automatic U.S. involvement in terms of the American homeland.

Other argue that if an American atomic weapon explodes in the Soviet Union, the Soviets will not care where it was lauched from but would retaliate against the U.S. mainland.

Supporters of these new weapons argue that, in the last analysis, they are needed to plug a gap in alliance defenses and that the West needs the ability to respond in kind to any level of Soviet attack. Such a capability is the thing that best provides the continued balance of power and mutual deterrence to war, they say.

The new Pershing and cruise missiles are not meant to knock out the Soviet SS20 missiles, which are mobile and hard to locate and strike. Rather they are meant to give NATO the ability to also strike quickly and deep at important Soviet military targets feeding a European war effort.

Among the casualties of the protracted and politically strained debate in recent months has been the image of Dutch contribution to Western defense. i

The Dutch have been criticized for their stance, yet the Dutch position has clearly brought at least an opposition view into the debate. And, while the Dutch sensitivities to nuclear weapons appear to go beyond those of other allies, the Dutch have continued to maintain their conventional military forces and are one of the few NATO countries that has lived up to the pledge to increase defense spending by 3 percent.

The Dutch recently ordered 450 new tanks from West Germany to modernize their land forces.