For several years, the Navy had planned to close a small base on the Caribbean island of Antigua that offered a pleasant tour of duty but no longer served much military purpose.

Then, after the United States launched what officials call a "rescue operation" on neighboring Grenada in 1983, plans changed. Both the government of Antigua and Barbuda, a dependency of Antigua, and officials working for Defense Secretary Caspar W. Weinberger objected to closing the base. Therefore, the Navy plans to find a new mission for the eastern Caribbean facility and to invest $8 million in it during the next several years.

The episode, although involving little money by Pentagon standards, reveals how intricate questions of foreign relations can complicate any effort to reduce overseas commitments, which now keep more than 500,000 military personnel abroad.

"We thought east Caribbean nations would take this as a sign of diminished presence and commitment," said Dov S. Zakheim, deputy undersecretary of defense for planning and resources. "In the aftermath of Grenada, that was not a signal we were trying to send."

Lionel Hurst, first secretary of the Antiguan embassy here, agreed that his government considers the base a deterrent to "bandits" who might seek to destabilize the island nation. He also said that U.S. "renumeration" for use of the base -- about $1.5 million annually, according to U.S. officials -- is important to Antigua.

As a result of the geopolitical considerations, the military has found a new use for the Antiguan facility. The camp will be used by special forces and other troops from all services for SERE training -- schooling in survival, evasion, resistance and escape.

"Right now we have no tropical SERE training site in the Atlantic Fleet area," Zakheim said, adding that the current SERE site is in Maine. "Antigua has cactus deserts, swamps, jungle, deep water, shallow water. . . .You try to find some jungle in Maine."

U.S. forces do train in jungles in Panama and, more recently, Honduras, but those areas come under the Southern Command, not the Atlantic Command.

The U.S. military has operated a facility in Antigua since 1941, when the British traded the post and several others in exchange for 50 U.S. destroyers. When Antigua gained independence from the British in 1981, William P. Clark, then deputy secretary of state, warned the new nation of Cuban subversion and promised to maintain the U.S. base.

But improvements in technology made the Navy mission there obsolete. Although the post was known as an "oceanographic research" station, it was used by the Navy as a listening post for detecting foreign submarines. Equipment with longer ranges has reduced the number of such stations needed.

The Air Force also maintains a small ground station for satellite communications on the island. Zakheim said the expense of relocating the Air Force personnel, who now share Navy space, would have roughly equaled the eventual savings from closing the Navy post.

Antiguan representative Hurst suggested that the U.S. presence in the area will help provide security for President Reagan's visit to Grenada next month.

But Hurst said the island nations of the eastern Caribbean have been less receptive to U.S. prodding for the creation of a regional security force. "We need more teachers and more schoolrooms and more schoolbooks," he said. "Our primary concern is economic development and only secondarily in defense and military machinery."

The Navy plans to station about 70 people on the island, with other units rotating in for training.