Across Western Europe, defense-minded governments that took power determined to blunt the Soviet threat are finding that they cannot pay the higher bills in military spending urged by the Reagan administration.

Belgium's center-right government has informed allies in the North Atlantic Treaty Organization that it cannot afford its share in the modern Patriot antiaircraft system, a default that would leave a gaping hole in the alliance's integrated air defense network.

Britain has just slashed $360 million from its defense budget and is seeking ways to curtail its tasks within NATO. The British Navy has spurned appeals to play an expanded role in the Mediterranean because its fleet is overburdened by the defense of the Falkland Islands.

The West German Army is anticipating an acute manpower shortage caused by a falling birth rate and a sharp increase in conscientious objectors. Last year, 60,000 West Germans, or one out of every six youths eligible for the draft, refused to serve in the armed forces because of antiwar convictions.

Last autumn, the Danish parliament voted to cancel the country's share of NATO's infrastructure costs for deployment of Pershing II and cruise nuclear missiles in five other European countries at the end of this year. The conservative minority government opposed the parliament's action. Denmark also has restricted the defense budget to a 0.5 percent increase this year. That is far short of the 3 percent annual increase, after inflation, that NATO members prescribed as a common goal for military spending in 1977.

Indeed, Europe's prolonged recession has compelled most governments to abandon the 3 percent mark as an impossible goal. Among NATO members, only Norway and the United States will increase defense spending by more than 3 percent this year.

"From our standpoint, the political situation within the alliance is the best it has been for more than a decade," explained a NATO policy planner. "But all of these governments are finding it extremely difficult to come up with the money that the military wants."

Recent elections have swept conservative governments to power in London, Bonn, The Hague and Brussels, creating what Washington perceived as a more sympathetic political coalition to invigorate Europe's defenses after what were deemed as years of neglect.

Even the Socialist-led government in France, a country that pulled out of NATO's military command in 1966, unveiled plans for major improvements in the country's independent nuclear deterrent system upon taking office two years ago.

Yet, at a time when Europeans are absorbed by the controversial deployment of Pershing II and cruise nuclear missiles later this year if arms talks fail, a more fundamental crisis is emerging from the grim recognition that an era of declining prosperity is starting to erode Western Europe's defenses.

No matter what their ideological complexion, European governments are confronting the fact that austerity defense budgets mean that pilots will receive less training, ammunition stocks will be lower and plans for more sophisticated weapons will be postponed if not scrapped.

"It now looks like it will be extremely difficult to maintain current military structures," said a West German planner at NATO headquarters. "For some countries, real increases in defense spending will be nearly impossible for the next five years."

The European spending squeeze lies at the heart of important defense dilemmas, to be examined in this two-part series, that are raising risks of further alienation between the United States and Europe.

When NATO was created in 1949, the United States reigned as the world's dominant economic and military power, and its protection was coveted by Europeans still struggling to rebuild from the ruins of war.

Today, at least six European nations have higher per capita incomes than the United States, and the Reagan administration, backed by Congress and public opinion polls, has urged the Europeans to spend more for their own defense.

European NATO members have responded with a report contending that they supply 80 to 90 percent of NATO land forces, airplanes and tanks along with 70 percent of the alliance's warships.

They say that additional defense burdens would cripple their economies, weakened by four years of recession. The prospect of social unrest caused by mounting unemployment and welfare cuts, it is argued, also pose security risks that must be contained.

The Europeans have also contested U.S. requests to defend western interests through what are labeled "out of area" commitments.

The American strategy to defend the Persian Gulf oil fields will require substantial allied help in providing transit facilities while picking up the defense slack on the central European front if U.S. troops were suddenly dispatched to Middle East trouble spots.

But European governments have balked repeatedly at the concept of extending NATO's defense perimeter. Their reticence has exasperated American defense planners who believe that the Europeans are more dependent on Persian Gulf oil and should not expect the United States to bear all the costs in protecting their energy supply channels.

The simmering conflicts over budgets and regional responsibilities are intensifying just when the alliance is confronting the threat of protests this fall against the stationing of new medium-range nuclear missiles.

The danger, as seen from NATO headquarters, is that demonstrations could assume anti-American overtones. Attacks on U.S. military outposts could once again inflame congressional demands to pull out some or all of the 350,000 U.S. troops based in Europe.

Beyond those immediate concerns, European reluctance or inability to spend more money on defense reflects to some extent differing assessments among the allies of the military threat posed by Warsaw Pact forces.

There is perhaps greater shared disdain toward the Soviet government between Washington and European capitals than at any time in recent memory.

The invasion of Afghanistan, the crackdown in Poland and the shooting down of the South Korean airliner with 269 people aboard have reinforced suspicions on both sides of the Atlantic toward the Soviet military establishment.

But while the United States has responded with a more resounding military buildup, European defense planners say their governments are not likely to bolster defense spending simply because in their view, the actual military threat has not changed dramatically.

As a defensive alliance, NATO has never tried to match the huge numbers of equipment and personnel fielded by the Warsaw Pact, but has focused its efforts on deploying an array of deterring forces capable of dissuading any Soviet strategist from thinking an invasion could be successful.

According to NATO estimates, for example, the Warsaw Pact possesses 42,500 tanks compared with 13,000 for the alliance. Yet, NATO planners and military officers at the supreme allied headquarters near Mons, concede that the West still holds the edge because the American-made M1 and West German Leopard-2 tanks are superior to the latest Soviet models, the T72 and T80.

Moreover, they said that tank personnel in the West are better trained and NATO's antitank defenses are more effective than rival systems in the East.

Communications problems and equipment breakdowns experienced by Soviet forces during the invasions of Czechoslovakia and Afghanistan have convinced some European defense planners at NATO that allied defenses will hold up much better than expected in the event of war.

The notion of a surprise attack from a "standing start" has been discredited in NATO intelligence circles because the alliance would have weeks of warning that nefarious plans were stirring in the East.

"We would detect very quickly movements that predate an attack by about three weeks," explained a NATO defense planner. "Troops would have their leave canceled, submarines would be departing bases en masse, any number of things we would see well before D-Day arrived."

Soviet military strategy dictates that Warsaw Pact forces should probe for weak links in western defenses and then attempt to pour waves of troops followed by fresh reinforcements through the breach.

A major weakness of the Soviet military structure, however, is exposed by its rigid and centralized command system that offers little initiative or flexibility to officers in the field.

NATO planners say this rigid chain of command, as well as ponderous supply channels, provide numerous targets at choke points where the West could disrupt an advance from the East.

The most glaring shortcoming, however, remains the unreliability of East European forces in any offensive thrust conducted by the Soviet Union. NATO officials admitted that it is difficult to believe armies in Poland and Hungary would "dutifully march in lockstop" with the Soviets.

Gen. Bernard Rogers, NATO's supreme military commander, is far more impressed with Warsaw Pact capabilities and states flatly that if an invasion occurs, he will be compelled to ask his political overseers for authority to use nuclear weapons "fairly quickly" to avoid being overrun.

"We have mortgaged our defense to the nuclear response," he said. "The plain fact is that if conventional war comes, we will simply be unable to sustain our forces for long with manpower, ammunition and war reserve stocks."

Rogers could proceed to use American nuclear weapons in Europe with the permission of the U.S. president, but he would face a much more apprehensive response in approaching European allied leaders.

"If he ever asks for such authority to deploy nuclear weapons, the chances of him getting it are almost nil, unless the situation is impossibly out of hand," predicted a NATO policy planner.

Next: The Nuclear Threshold