Washington — American troops and their Caribbean allies appear to have had a relatively easy time invading the tiny island of Grenada. The announced intention of the Reagan administration is to evacuate American citizens on the island and then withdraw as quickly as possible, leaving the task of building a new Grenadan government to regional allies and to the Grenadans themselves. That task may prove to be considerably more difficult than launching the invasion.
Grenada has been under the control of left-leaning military coup leaders since Oct. 13.
For Americans, the US involvement in Grenada appears to be as different from the involvement in Lebanon as it could possibly be. Grenada is a Caribbean island of only 110,000 people, whose leftist government is cut off from allies who could help it resist the invading forces. Only 21 miles in length and barely a dot on most maps, Grenada has been noted mainly for its nutmeg and its beauty - and most recently for its close ties with Cuba. It boasts of white beaches, green mountains, hibiscus flowers, and mango trees.
In contrast with the Lebanon situation, where opposition from Americans to the United States military involvement is widespread, the invasion of Grenada is expected to trigger much more limited public criticism. Senate Foreign Relations Committee chairman Charles H. Percy, a Republican from Illinois, said Tuesday that none of the congressional leaders from either party expressed opposition to the invasion when they were informed by the President at a secret White House meeting Monday night.
House Speaker Thomas P. O'Neill Jr., a Democrat from Massachusetts, said he did not intend to criticize the action at this time. Mr. O'Neill said that congressional leaders were not asked for advice.
But once the invasion was announced publicly, several congressmen did raise questions. One asked whether US forces were embroiled in too many conflicts. Some prominent members of the American black community who have been concerned about Grenada for some time asked whether the invasion could set a precedent for similar US actions elsewhere in the world. And questions were raised by Charles Modica, chancellor of St. George's Medical College in Grenada, who is currently in the United States. Chancellor Modica told NBC News that American medical students on the island were not in danger and could have been evacuated in an orderly manner. He charged that by launching the invasion, President Reagan had taken ''some very unnecessary risks.''
In announcing the invasion on Tuesday, the President listed as the first and most important of three reasons for the action the protection of innocent lives, including those of up to 1,000 Americans on the island. The second and third reasons, Reagan said, were to forestall further chaos on Grenada and to restore order and institutions. The President charged that the island nation's prime minister, three of its cabinet ministers, and two labor leaders and other civilians had been killed by ''a brutal group of leftist thugs.''
Reagan was referring to the killing by Grenadan troops of Prime Minister Maurice Bishop and others last week after Mr. Bishop's government was ousted. A suave London-trained lawyer with leftist leanings, Bishop had taken power more than four years ago through a coup of his own. Although he was close to Cuba's leader Fidel Castro, Bishop had apparently decided some time ago to distance himself from the Cubans and to seek a rapprochement with the United States.
An administration official said that in a meeting with Reagan's then national security adviser William P. Clark in Washington some weeks ago, Bishop had spoken of the need to build a ''more constructive relationship'' with the US.
Bishop's opening to the US was apparently among the trends which disturbed those in the Grenadan government and army who moved against him.
By participating in the invasion, the US reassures Caribbean allies who were concerned about the leftward turn of Grenada. Ever since the island's leftist-led coup of 1979, the concern of the US and those allies has been that Grenada might become a Cuban surrogate and training ground for insurgents. The US was also concerned that Grenada's international airport, which was being extended with the help of the Cubans, might serve as a refueling stop for airplanes carrying Cuban soldiers to Africa. It is also close to important oil-shipping lanes.
Although it was not part of the officially announced purpose of the invasion, the action sends a signal to Nicaragua that the US is willing to intervene directly and with force in the region when it considers it necessary. Nicaragua Tuesday requested an urgent meeting of the United Nations Security Council to consider the invasion of Grenada.
Some 2,000 US marines and soldiers carried the brunt of the invasion, with six Caribbean nations contributing some 200 soldiers to the invasion force. Grenada's army numbers some 2,200 men. Radio Havana said Cubans on the island, estimated at about 600 persons, had fought ''heroically'' against the invaders but were surrounded and suffered losses. Defense Department officials said American losses were ''minimal.''
President Reagan said that the US had participated at the request of five member nations of the Organization of Eastern Caribbean States.
Congressman Michael D. Barnes, the Maryland Democrat who chairs the House subcommittee on Western Hemisphere Affairs, said he would withhold final judgment. But Mr. Barnes wondered whether US citizens on Grenada were really endangered, asking: ''Or is this just the excuse the administration was looking for to go in there anyway?''
Rep. Edward J. Markey (D) of Massachusetts labeled the invasion a new edition of ''gunboat diplomacy.'' And Rep. Bill Alexander, Democrat from Arkansas and House deputy majority whip, said ''the President relies too heavily on confrontation and the use of force as a substitute for foreign policy.''
Staff writer Julia Malone contributed to this report.m