THE HAGUE -- When the Reagan administration began its military build-up in the Persian Gulf, its European allies refused to contribute forces and criticized the U.S. policy as provocative and ill-advised.

Now, four months later, almost all of the same governments have dispatched warships to join U.S. forces in the region, taking part in what officials say will be a 70-ship concentration of allied naval power, the largest since the Korean War 35 years ago.

What happened between then and now, European officials say, was a chain reaction of national decisions, made against a background of U.S. pressure, that produced the political result the European governments originally had been trying to avoid.

As a result, the Reagan administration has gained the symbols of international backing that most Europeans had rejected earlier as unwise.

The turning point came Aug. 10, when the U.S.-operated oil tanker Texaco Caribbean hit a mine off Fujayrah in the Gulf of Oman, about 80 miles south of the Strait of Hormuz.

The Panama-registered ship, which was left with a 12-foot hole just below the waterline, was the first craft to encounter mines outside the Persian Gulf.

The explosion created new concern about the U.S. operation in the region, since it seemed to widen the danger area. But it particularly set off alarm bells in Paris and London, because both French and British military ships were using the waters south of Hormuz as a staging area.

The next day, France and Britain announced almost simultaneously that they were sending mine sweepers, with support craft, to the Gulf of Oman -- a gesture both had refused to make in May, when Defense Secretary Caspar W. Weinberger asked them publicly.

According to U.S. and European sources, the two had also refused one week earlier, when National Security Adviser Frank Carlucci asked them privately.

The French and British decisions surprised the Reagan administration, which learned of them shortly before they were announced, these sources said. "It just fell out of a tree," said one U.S. diplomat.

They also surprised other European governments, which until then had thought they were part of a united policy of maintaining distance from U.S. actions in the gulf, several European officials said.

The French and British governments emphasized that their mine sweepers were dispatched only to protect French and British ships, not to work with U.S. forces.

According to several European officials, the decisions were made primarily on the basis of judgments that the mine sweepers were necessary militarily, not in response to U.S. prodding.

France, locked in a confrontation with Iran, also had its own reasons for a muscular policy in the area, a French official pointed out.

But both countries' military leaders were concerned about the danger the mines posed to their naval forces, gathered in the Gulf of Oman between expeditions into the gulf.

At the same time, the Netherlands had been exploring the possibility of a U.N. or joint European force with the other six members of the Western European Union.

Dutch Defense Minister W. F. van Eekelen, in an interview with Washington Post correspondent Karen DeYoung, said he had favored "in principle" responding to Weinberger's request from the beginning, but as part of a European effort.

This was important because in Dutch eyes, a European force meant that participating nations "can't be singled out for raising tension and being provocative," he said.

In addition, van Eekelen said, a cooperative European effort would allow the participants to share logistical support and air cover.

A European or U.N. force also would have made possible a more visible separation from U.S. operations for the benefit of European public opinion and Iran, officials here pointed out.

But the Dutch contacts were eclipsed by the French and British decisions on Aug. 11.

Dutch officials said that as a result, they felt increased pressure for action by the other European nations, which had been left in the position of refusing a U.S. request while Europe's two major military powers went forward.

In that atmosphere, top-level defense and foreign affairs officials of the seven Western European Union nations met in The Hague on Aug. 20 to seek a common policy.

Although they agreed to declare that freedom of navigation in the gulf was part of their "vital interests," they could not settle on joint action because Britain and France insisted on acting individually.

The Netherlands was seeking a formula for a joint effort, Dutch officials said, because the government wanted to make a positive response to the Reagan administration as a loyal Atlantic ally, but did not want to act alone.

The Netherlands felt particularly called on to do so because it has advanced mine sweepers and because Britain and France, for their own reasons, already had moved.

In the background was a Reagan administration proposal, transmitted through the U.S. Embassy here, calling on The Hague to position mine sweepers in and around the Red Sea as a first step toward eventual deployment nearer the gulf, van Eekelen said.

Instead, the Netherlands and Britain worked out what Dutch officials called "technical coordination" between their forces.

In practice, they said, this meant British aircraft would provide protection for Dutch mine sweepers and British logistics vessels would provide other support within the British operational area in the Gulf of Oman and inside the Persian Gulf, as far as Bahrain.

Armed with this accord, the Netherlands announced Sept. 7 that it was sending two mine sweepers to the gulf region.

Belgium, which traditionally has worked closely with the Netherlands in military affairs, quickly followed suit, dispatching two mine sweepers and a support vessel.

The two nations' naval units will be commanded by a Belgian officer on the scene, and a Dutch officer will have overall command of the joint force here, Dutch officials said.

Liaison officers with the British command in the gulf will help coordinate operations assignments, logistics support and intelligence sharing, they added.

Although separate from the Americans, this coordinated command is likely to work closely with the U.S. command as well, they acknowledged.

"On a naval level, if you don't have close coordination, you are mad," van Eekelen said.

The level of coordination among U.S., British and French ships has not been disclosed. But similar practical considerations appear likely to make close contact necessary despite the political desire in European capitals to emphasize separation.

Italy, whose then-prime minister had responded to Weinberger in May that "we are not the Marines," also was moving fast toward a decision to deploy naval forces by early this month.

The likelihood that Italy would send mine sweepers toward the gulf soon spurred the Netherlands and Belgium to move ahead more swiftly than planned, European officials said.

As the government argued over the issue in Rome, an Italian container ship was fired on in the gulf Sept. 8 by a small boat that Italy identified as Iranian. The next day, Rome announced the dispatch of eight ships, including three mine sweepers and two frigates.

{Four Italian vessels arrived Saturday in Port Said, Egypt, en route to the Suez Canal, Reuter reported, and four others are due Sunday.}

West Germany, citing constitutional restrictions, has remained firm in its refusal.

Some officials have suggested that West Germany instead could fill in for any NATO forces sent to the gulf.

Even without West Germany, the result of the European decisions is that about 35 U.S. and 35 allied warships are scheduled to be operating in and around the gulf by the middle of next month.

The allied fleets will join a half dozen Soviet Navy vessels and the navies of Saudi Arabia, Oman, Iraq, the United Arab Emirates -- and Iran.

Wary of U.S. policy in the gulf, which they judged needlessly confrontational toward Iran, the allied governments nevertheless moved step by step into at least indirect association with it -- and now they are wary of the uncharted consequences if U.S. and Iranian forces end up in hostilities.