Three decades after its founding, the Western alliance is at a political crossroad marked by America's preoccupation with conflicts outside Europe's own heightened sense of itself.

While the crisis in Afghanistan and Iran clearly have accelerated the process of rethinking within the alliance, the basic outlines of imminent changes have been emerging for some time.

An important turn comes this week at the ministerial meeting of the North Atlantic Treaty Organization.In a demonstration of post-Afghanistan anti-Soviet resolve, the 15 alliance members are expected to approve a speedup of NATO defense modernization plans.

But the session will be deeper significance for the accent it puts on Europe's increased contribution to the alliance.

"It's the beginning of the beginning" of a reformulation of the Western defense partnership, said an experienced NATO official. Just what this means, however, in terms of eventually redividing military responsibilities and ensuring political consensus in the alliance is still largely unexamined -- and for that reason, profoundly unsettling.

On the military side, the new situation is likely to translate into a considerably larger role for Europe in its own defense. On the diplomatic front, it will require NATO to pay greater heed to the Common Market political apparatus, used visibly by the Europeans in recent months to define policies on Afghanistan and Iran that differed in part from America's.

Part of the difficulty is assessing the changing order is that the allies themselves have avoided any deep discussion of it. Much of the inter-Alliance squabbling over the past few months has been about matters of style -- about surface considerations such as whether consultations on Iran were adequate, whether America over-reacted, whether Europe shirked, whether President Carter meant what he said or did what he meant or said what he did, and so on.

The disagreements among allies have been excused as basically matters of tactics, not vital issues. But the stream of verbal assurances about enduring U.S.-European friendship and common Atlantic purpose have not washed away the keen sense that Western Alliance relations need to be placed on some new footing. How to do this remains the central problem.

Of course, there have been trans-Atlantic highs and lows previously. One tendency evident among Western diplomats and European politicians now is to place the latest tensions on a par with the others -- and to say these too shall pass. The analogy of marriage squabbles that come and go has become a favorite way to explain the NATO quarrels.

At the same time, commentators assert that the recent tussles are putting the alliance under strains which arguably are worse than at any point in NATO's 31-year history.

Moreover, the differences on Afghanistan are set apart from previous Alliance strains -- such as Vietnam, Palestine, oil, nuclear nonproliferation, trade and the dollar -- in that the current crisis addresses NATO's cornerstone concern: how to respond to the Soviet threat. a

NATO sources say that rapprochement, at least, has been achieved between what were originally differing national viewpoints on why the Soviets did what they did.

There is also reportedly common agreement within NATO that the Afghanistan invasion, while outside the NATO treaty area, constitutes a serious threat to East-West relations and requires a joint alliance response. s

But the question of how to respond has been further complicated by America's pledge to defend the Persian Gulf.

The key strategical concept for European defense has been what is called "flexible response" -- meaning that in the event of an attack on Europe, America would airlift troops from the United States to reinforce fighting units here. This made Europe dependent on troops it did not have, and was based on the assumption that U.S. forces would be available and could get here in a hurry.

U.S. officials say there are no plans to withdraw American divisions already stationed in Europe. Even so, the possibility of fewer available U.S. reinforcements is causing a reassessment of alliance defenses.

Detailed planning has only just begun, according to NATO sources. Defense ministers are expected to discuss a number of U.S.-proposed measures at the NATO meeting this week. These include:

A build-up of Europe's own low-level munitions reserves.

Intesified training of European reserve units:

The readying of European civilian airliners to convert on short notice to military use to help ferry military troops;

An increase in the amount of war equipment prepositioned in Europe.

Specifics differ among European nations. Overall, though, they add up to what NATO defense planners expect will be only the first phases of an increased European contribution to the alliance.

The steps proposed so far contain little that is really new to the Europeans. Most of these measures were outlined in the long term defense program approved by NATO in 1978 to shore up alliance defenses. What the United States has pressed for since the Afghanistan invasion is an adjustment in some program priorities and assurances that alliances members will live up to past commitments and accelerate others.

So far, Europe has voiced a general willingness to do so. Touting "division of labor" European officials have said they will accept a greater defense burden within NATO in return for expanded U.S. protection over vital oil supplies in the Persian Gulf.

But no one seems in any hurry to define what exactly the new divisions will be. "Europe is in a real turmoil over this," said one knowledgeable NATO official. "They don't seem to want to adjust to the United States having a competitive military interest outside Europe."

European defense officials say they are not able to expand their own military efforts much in the near-term any way, both for lack of resources and money. Moreover, they note, Europe already carries more of the NATO burden than Americans seem to realize.

As for joining the United States militarily outside NATO, that remains officially an individual country matter. NATO sources say there was never any serious discussion in alliance councils about extending the NATO area beyond its current limit.

This notion has long been unacceptable to NATO's northern flank members, unconscionable for the West Germans, and anathema besides to some of the countries that would find themselves included in the new boundaries.

At the same time, European officials do worry about the security of their oil sources. While NATO, as such, may not be able to operate outside its area, forces of individual European nations surely can and will.

The navies of France and Britain are expected to be showing their flags in the Indian Ocean more frequently along with the U.S. fleet. Other member nations are supplying economic and military aid to the Gulf region, again without pinning a NATO label on it.

But as one West German official explained, the real guarantee to peace in the Middle East is seen here not as a military problem but rather dependent on a diplomatic resolution of the Arab-Israeli conflict.

In the making of foreign policy, it is evident the European nations increasingly prefer to do their thinking in the context of the Common Market.

Again, this development is not new. But European leaders quite aggressively used the twin crises of Afghanistan and Iran to bolster a political directorate of their own distinct from NATO's. Europe's positions on a trade embargo against Iran, economic sanctions against the Soviet Union and boycott of the Moscow Olympics were all crafted chiefly in the councils of the European Economic Community.

The question: Will a more united European community likely facilitate the informulation of a common U.S.-European political conception on joint problems, or will it lead to a strong rival European faction within the Western alliance?

In the past, transatlantic political difficulties have not affected the military utility of NATO. Since Afghanistan, however, the European allies -- particularly West Germany and France -- have placed stronger emphasis on the continuation of detente and the pursuit of arms control negotiations than has Washington, though officials on all sides deny there is a substantive policy difference.