Like many good espionage mysteries, France's "Greenpeace affair" began with a simple bureaucratic note.

The note was written six months ago -- on March 1 -- by the head of the French nuclear testing center at Mururoa Atoll in the South Pacific, Adm. Henri Fages.

France, the admiral urged, should step up its intelligence-gathering efforts to "anticipate" a planned protest campaign by the Greenpeace environmental organization against French nuclear tests.

The significance of the verb anticiper, underlined twice in the memo, lies at the heart of the most embarrassing political scandal yet faced by President Francois Mitterrand's Socialist government.

Was the choice of the word, which carries a vague connotation in French of "acting to forestall," insignificant, as Fages now maintains? Or was it a deliberately ambiguous instruction that set in motion a disastrous chain of events culminating in the sinking of a Greenpeace ship and the death of a Portuguese-born Dutch photographer on the other side of the world?

The answer to these questions must be sought in a series of tantalizing clues that have turned up after the sabotage of the Rainbow Warrior in the New Zealand port of Auckland July 10.

Alternative solutions have been offered to the mystery.

One is the legal case laboriously being put together by New Zealand police, who have arrested two officers of the French secret service and charged them with murder, arson and conspiracy. The other is an official report by a French government investigator acknowledging that agents of the General Directorate for External Security (DGSE) were in New Zealand to spy on Greenpeace but clearing them of the crime against the Rainbow Warrior.

Ironically, one of the principal arguments in favor of the innocence of the men from la piscine (the swimming pool), as the General Directorate is known here, is the wealth of incriminating evidence against them. So Gallic is the trail, observed a directorate source sarcastically, that the only missing clues are a baguette bread loaf, a black beret and a bottle of Beaujolais.

Could the "swimming pool" really have bungled so badly?

Adm. Fages had every reason to be angry about the Greenpeace plans, which reportedly included the idea of escorting boatloads of French Polynesian separatists toward Mururoa.

The French military long had regarded the environmental organization with suspicion, even loathing, believing it to be infiltrated by Communists and Soviet spies opposed to France's independent nuclear deterrent, known as the force de frappe. It was partly in response to a previous Greenpeace campaign that French nuclear tests were moved underground in 1975, a much more expensive and cumbersome procedure than holding them in the atmosphere.

The official French investigator, Bernard Tricot, has conceded that news of the resumption of Greenpeace protests provoked considerable "irritation" in Paris. But he has cited government documents indicating that plans for dealing with the organization this time were exactly the same as in previous years, namely deploying the Navy to prevent the protesters from entering French territorial waters.

In fact, there is evidence that the idea of sabotaging a Greenpeace ship had been considered by some French military officers in the past. According to Bernard Stasi, a former minister for France's overseas territories, the military almost succeeded in getting such a plan approved in 1973 but was overruled by the politicians.

The Fages memorandum was followed by a meeting between Defense Minister Charles Hernu and the head of the secret service, Adm. Pierre Lacoste. Hernu now says he merely instructed the service to "observe" and "infiltrate" Greenpeace. As Lacoste remembers the conversation, the defense minister also agreed to let the agents "reflect on ways and means to counteract" Greenpeace initiatives. Tricot, on the basis of the testimony of senior secret service officials, insisted that there was never any question of authorizing direct action against the Rainbow Warrior. He does not explain in his report, however, why four of the six French agents later sent to New Zealand belonged not to the directorate's "research" division, but to its "action division," the "James Bond" wing of la piscine.

The government has refused to reveal how much the Greenpeace operation cost, but it has been unofficially calculated at around 3 million francs, or $400,000. This was more than the directorate's budget could bear: financial approval for the operation, according to Tricot, had to be obtained from Gen. Jean Saulnier, President Mitterrand's chief military aide.

The first French agent to show up in New Zealand in late April was Lt. Christine Cabon, a member of the directorate's research staff. Before leaving France, she had joined the ecological organization, the Friends of the Earth. Posing as Frederique Bonlieu, a geologist opposed to French nuclear tests in the Pacific, she infiltrated the Auckland branch of Greenpeace.

Cabon left Auckland on May 24, her mission apparently accomplished. Useful information that she supplied to the directorate included a map of Auckland harbor later found in possession of one of the agents. She was last heard of at an archeological dig in Israel in mid- July, from which she sent a postcard to her old "friends" at Greenpeace deploring the sinking of the Rainbow Warrior.

Back in France, meanwhile, two more teams of agents were preparing to be sent to New Zealand. Each was headed by a trained combat frogman and member of the directorate's action division: Maj. Alain Mafart and Master Sgt. Roland Verge.

Senior directorate officers interviewed by Tricot insisted that the two teams had entirely different missions in New Zealand and were unaware of each other's existence.

Verge's team arrived in a yacht, the Ouvea, from the French Pacific territory of New Caledonia. Its mission, according to the Tricot report, was to watch the Greenpeace fleet gathering in Whangarei harbor, in northern New Zealand, acquire sailing experience and investigate the possibility of joining future protest flotillas. Mafart, on the other hand, was responsible for watching the Rainbow Warrior, the Greenpeace flagship, in Auckland harbor, and identifying Greenpeace sympathizers. He was accompanied by Capt. Dominique Prieur, and they used the names Alain and Sophie Turenge.

The New Zealand police contend that the two teams of French agents were working closely with each other in a coordinated plan to sabotage the Rainbow Warrior. New Zealand prosecutors are expected to attempt to prove in court that the explosives used to sink the Greenpeace ship were smuggled into the country on board the Ouvea and planted on the hull of the Rainbow Warrior either by Mafart or under his supervision.

There is strong circumstantial evidence that the French agents were in contact with each other. Mafart is Verge's deputy commanding officer at the directorate's center for combat frogmen in Corsica. Both teams arrived in New Zealand on the same day, June 22. There are some remarkable coincidences in their itineraries. It is difficult to see why Mafart spent July 5 through 7 at Paihia if his principal task was to observe the Rainbow Warrior in Auckland, nearly 200 miles away.

Questioned by journalists about evidence of contacts between the French agents, Tricot replied: "These are troubling facts. If it can be proved that these contacts did take place, it would be very serious. It would show that I have been lied to on an important point."

In his report, Tricot cited the ostentatiously carefree attitude of the French agents -- particularly the Ouvea crew -- as evidence of their innocence. It is true that, if they really did blow up the Rainbow Warrior, the men from the "swimming pool" can be described as blundering incompetents.

The trail left behind by Verge and his two colleagues includes an entry in the guest book of a pizzeria in Whangarei that they visited in the company of a New Zealand woman called Carole. Among the clues found by the police after the sabotage of the Rainbow Warrior were French-made oxygen bottles. As Tricot pointed out, one would have expected French saboteurs to have taken the elementary precaution of using easily obtainable foreign oxygen bottles for such an expedition.

On the other hand, the behavior of the French agents is also hard to square with their ostensible mission in New Zealand. It is strange that none of the directorate men aboard the Ouvea spoke more than pidgin English, a serious drawback if they were meant to "mix with the local population," as Tricot maintained. And for a team that was meant to be gathering sailing experience, it is hard to explain why they traveled more than 1,000 miles by car around New Zealand's northern island.

The Ouvea left Whangarei on July 9, a day before the explosion aboard the Rainbow Warrior. Greenpeace officials have asked why Verge's team should have left New Zealand on precisely this date if its mission was to observe a protest flotilla that was only due to depart at the end of the month.

The answers to many of these questions will have to await the trial of Mafart and Prieur, the "Turenges," which opens in Auckland Nov. 4. In the meantime, international arrest warrants have been issued for Verge and the two other French agents aboard the Ouvea.

Whatever the eventual outcome of the trial, the immediate result of the affair has been the opposite of what the General Directorate wanted to accomplish by sending its agents to New Zealand. Greenpeace has won more worldwide attention over the past six weeks than ever before in its 15-year existence.