Europe tries to encourage Arab moderates to step into breach

''It's bad - but Syrian gains could be in jeopardy in coming months, and there are other developments in the Mideast that may limit the damage.'' That's the stiff-upper-lip way US allies in Western Europe say they view the latest decisions to move United States and British peacekeeping forces from land to sea.

None of the allies wants to publicly overplay the scale of Syrian, and indirect Soviet, gains in Lebanon.

None wants to enter a public quarrel with President Reagan at the moment, even though privately, officials in London and Paris fault US policy for aligning itself too strongly with Lebanese President Amin Gemayel.

Instead, both London and Paris officials make comments designed to limit the damage already done in Lebanon.

They point out that Syria's President Hafez Assad has a number of internal and external problems of his own to worry about. His economy is in poor shape, he faces internal opposition, he is at odds with neighboring Iraq, and he fears the spread of Iranian fundamentalism into Lebanon.

This is the time, European officials feel, for moderate Arab opinion to exert itself. They note Egypt's return to the Islamic Conference, moderate PLO reconciliation with Egypt, and Jordan's King Hussein preparing to meet with Yasser Arafat.

Europeans concede that Syria, and indirectly the Soviet Union, have for the moment at least gained in Lebanon.

While Britain and France still say they cling to their hopes of seeing United Nations forces take over in Beirut, officials concede that President Reagan's decision to move his 1,600 marines to ships off Beirut has dealt their hopes a heavy blow.

Nor is there any clear picture of what will happen now to President Gemayel.

At this writing, Britain's 100 Queen's Lancers had been taken by helicopter to a fleet auxiliary ship, the Reliant, off Beirut.

Italian Defense Minister Giovanni Spadolini had announced in Rome that Italy was preparing to ensure the safety of Italy's 1,500 troops by following measures ''equivalent'' to British and American decisions. This appeared to mean redeployment to ships or other points near Beirut.

The defense minister had also said Italian forces would continue to guard the Sabra and Shatila Palestinian refugee camps.

Only France had not announced a shift of its own 1,200-man force from Beirut.

Observers in Paris, however, felt that while the French government might delay a decision to try to distance itself from the Americans, a redeployment was virtually inevitable.

''It's probably impossible to continue,'' a senior French official told this newspaper in Paris. ''We'll probably have to withdraw.''

French faces were gloomy Wednesday.

The Mitterrand government said President Reagan had not consulted it on US redeployment but had merely ''informed'' it. It seemed to Paris that the US was using a smokescreen of rhetoric to mask its moves.

The rhetoric included promises to keep training the Lebanese army and to shell positions controlled by the Syrians.

On the political side, the French feel the US has erred by not pressing Damascus and Moscow harder to arrange an international solution in Lebanon. Also , it was unreasonable to expect to maneuver in Lebanon without modifying the May 17 agreement between Lebanon and Israel, which Syria flatly opposes.

The agreement called for the withdrawal of most Israeli troops and political and economic cooperation.

By backing Gemayel so strongly, the French say, the US converted the multinational force from a neutral to a committed body, and undercut its usefulness.

British officials in London said their decision to withdraw the 115 British troops to the ship Reliant was taken because there was no longer a government for the British troops to help protect.

The British contingent was in an exposed position. London had consulted with Washington, Paris, and Rome.

The decision to pull out was made early Feb. 7 by Prime Minister Margaret Thatcher. Officials say she made it independently of Mr. Reagan, but it is clear she acted in the knowledge that US marines were to be removed.

One feeling in London is that Reagan's policy handed Damascus only two options: negotiate the Americans out, or force them out. President Assad chose the latter.

It is recognized that both Assad and the Soviets can block any UN peacekeeping force. Syria demands a favorable internal settlement in Lebanon first. The Soviets have a veto in the Security Council.

Meanwhile, some officials in Paris saw no alternative to a continued, intensified, partition of Lebanon, with the Israelis in the south, the Syrians in the east, the Christians in the middle, the Muslims in west Beirut, and the Druzes in the Shouf mountains.