West European governments are again being forced to rethink their view of President Reagan as an ally and as head of the world's most powerful country. The President's decision to send marines into the Caribbean island of Grenada is only one reason for renewed doubts about his competence and coolness as a world leader.
There is a rising belief in Western European capitals that Mr. Reagan has allowed himself to drift into a Middle East quagmire, with the United States commitment to the multinational peacekeeping force in Lebanon now openly questioned as a basis for future policy in the region.
In Paris, foreign ministers of the four countries contributing to the force were set for an urgent meeting to consider the future of the multinational force. US Secretary of State George Shultz was preparing for blunt indications from his British and French counterparts that the Europeans see no long-term future for the force.
Mr. Reagan's decision to brush aside Prime Minister Margaret Thatcher's objections to the Grenada invasion is seen by Britons as undermining the special relationship between London and Washington. Leading opposition and some government politicians are saying the President cannot be trusted any longer.
A startling aspect of these negative opinions is the speed with which they have developed.
In Western Europe Mr. Reagan's reputation as a world leader had been enhanced in recent months by the flexibility he had shown at the nuclear arms talks at Geneva.
Following the shooting down of the South Korean airliner by the Soviets, he gained more points by acting in a statesmanlike way and outclassing the Soviet leadership in his ''feel'' for the crisis.
Now suddenly renewed doubts are arising about sensitivity in dealing with potentially touchy allies and asserting US power in world trouble spots.