This distant piece of America in the Western Pacific more than 6,000 miles from California is again being invaded by the Japanese. Instead of the world War II helmets and khaki uniforms of 36 years ago, however, they wear colorful Hawaiian shirts. Instead of guns and swords, they carry cameras, shopping bags and bundles of yen. Since 1973, this island of 110,000 people has been saturated with up to 170,000 tourists a year - 99 per cent of them Japanese.

"This is our first foreign tour but things are so convenient. We feel we are still in Japan," said Yoshihisa and Hisayo Ishino, a young honeymooning couple from Shizuoka, near Mt. Fuji.

From the dining room menu to the instructions on "how to use the Western style bath," all hotel signs were written both in English and Japanese. The front desk staff and the telephone operator spoke Japanese. When the couple took a sightseeing bus, the tour guide chattered on in Japanese and all the passengers were Japanese. They even saw a Coca-Cola advertisement written in Japanese in a mountain area of the island. Kamaboko, a boiled fish paste; sembei, a rice cracker; other Japanese food and sake were available in a downtown supermarket.

They chose Guam for their honeymoon because it is relatively close to isolated Japan yet offers the exotic mood fo a tropical island. Guam is only three jet hours from Tokyo, and a 4-day package tour cost $500 per person.

"We like the sunshine, blue sea, hot climate, everything. We want to return with our children," said Yoshihisa> who has an electric appliance store in Japan. Behind the tiny beach where the couple lay in bothing suits, scarlet hibiscus and bougainvillea luxuriated in the sunlight. The calm, coral-filled shoreline was edged by white waves. Farther out, the turquoise color changed abruptly to navy blue.

For sightseers there are Spanish forts and bridges from the 16th century; for skin divers there is coral deep below the transparent sea; for culinary adventures there is local Chamoro food seasoned with coconut milk; and for just about everyone there are gifts at duty-free shop. In the arcade of one hotel, a pretty bride holding a list of 20 names was busily gift-shopping for her match-maker, relatives, flower arrangement teacher and friends.

The tourist age dawned in Guam in 1967, when Pan Am inaugurated jet flights from Tokyo. Since then, Guam's tourist industry has kept step with Japan's high economic growth. A dozen big hotels have been built on the shore, some with Japanese capital. Groups of Japanese office girls and white-and blue-collar workers vacation here, and the island has become a mecca for honeymooners. Japanese couples even get married in Guam to enjoy the romantic ceremony of a church wedding, though few Japanese are Christians.

Japanese tourists have been criticized for descending in mostly male hordes on South Korea, Taiwan and Southeast Asian countries. In Guam, where the visitors are well-behaved couples, the islanders' only complaint is the rate at which their coral is used to decorate Japanese homes.

Before the tourist invasion, Guam depended on the U.S. Air Force based and other government agencies for jobs and income. Agriculture is small-scale and ravaged by typhoons that sweep the island every year.

"This is why the tourism is so important for the island. That is the only way," said Martin Pray, general manager of the Guam Visitors Bureau. While government services annually bring in $300 million and remain Guam's main industry, tourism now injects and additional $100 million.

"Our tourism trade must be wholesome," said Pray, who describes Guam as a rare resort unmarred by gaudy night life. Therefore, he feels the ideal guests are families. He expects them to spend their vacations swimming, fishing, surfriding, camping in the jungle and playing tennis and golf. The islanders seem to hold the same opinion. In April they rejected by referendum a proposal for a gambling house on the island.

Although the Japanese lead the invasion of Guam, American visitors to the island have also greatly increased this year. From January to May, more than 10,000 Americans landed here, a 29-per cent increase over the same period last year. In the same period Japanese tourists increased only 5 per cent.

"I don't know why we had more American visitors," Pray said. "They used to be mainly businessmen and relatives of the workers at the base or with the government. Anyway, I hope this is a sign of our new relationship with the mainland. Guam is an ideal stopover point on the route to Asia. Americans car relax two or three days in an island setting before or after their whirlwind circuits of Asia."

The Guam Visitors Bureau expects TWA to begin a direct flight, adding another link with the mainland to the existing Pan Am service. To attract American interest, the Bureau is promoting the island as the center for package tours to other Pacific islands such as Fiji, Samoa, Tahiti and Tonga.

"The lifestyle in the island is now quickly adopting the American Way," explained one hotel manager. "The roads are paved, we have McDonald's and Kentucky Fried Chicken downtown. Everything is available in the supermarket. Guam isn't only for Japanese tourists," he said, grinning.