What began as a little-noticed request from the Kuwaiti government for U.S. military escorts for oil tankers has led to one of the largest peacetime buildups of U.S. combat forces since the Vietnam war.

"It's going to be very crowded out there by the end of this month," said one Pentagon official, describing the burgeoning U.S. forces being dispatched to the Persian Gulf and nearby Middle East waters.

In the past several weeks, the Defense Department has more than doubled the number of ships and forces assigned to the region, adding a battleship group, destroyers and frigates, helicopters and special forces units. By early September, the military expects to have about 31 ships and smaller vessels and more than 25,000 military personnel on duty in and near the gulf.

Part of that buildup, the battleship group led by the USS Missouri, which is expected to arrive in the Indian Ocean later this month, will provide a dramatic increase in firepower to U.S. forces in the area. The Missouri and her support vessels are among the most powerful and heavily armored ships in the U.S. fleet. Some Pentagon officials have discussed sending the six-ship group into the gulf, rather than leaving it in the northern Arabian Sea.

While the United States has amassed combatants in the Middle East and in other areas, such as Grenada, its most recent buildup is considered unusual because administration officials say it is intended as a defensive measure to protect shipping interests, not a sign of military aggression.

During the Iranian hostage crisis, President Jimmy Carter ordered two full aircraft carrier groups stationed in the northern Arabian Sea, putting a total of about 27 military vessels inside or near the Persian Gulf. Last year, a similar naval task force challenged Libya's "line of death" across the Gulf of Sidra in the Mediterranean.

Although it was the threat of Iran's military arsenal that prompted the Navy's buildup in the Persian Gulf region in preparation for the escorting operations, military officials say that in recent weeks they have turned their attention to the threats of terrorism and unconventional warfare.

The Navy, gun-shy over the mine explosion that damaged the tanker Bridgeton during the first convoy, and the unrelated Iraqi missile attack on the USS Stark that killed 37 sailors on May 17, has begun reassessing potential threats in the volatile region.

Military officials, responding to criticism that they have reacted to potential threats in the gulf on a crisis-oriented, piecemeal basis, have ordered new attempts to try to anticipate threats from Iran's most fanatical military groups.

Iran's Revolutionary Guards, a military group independent of the national defense ministry, is considered one of the most unpredictable threats. The guards on Friday completed four days of exercises simulating attacks on U.S. ships in the gulf and Strait of Hormuz, according to Tehran radio dispatches.

"It's very frustrating," said a Defense Department official. "We are reacting to our opponent who has control of the battlefield. We're put in a defensive position rather than orchestrating events ourselves."

Pentagon officials said Persian Gulf ship commanders in recent days have significantly increased requests to use conventional weapons in different ways to meet potential terrorist and other threats. The Navy, for example, already has met one plea to arm its small Seasprite antisubmarine-warfare helicopters with guns to attack terrorist or suicide speedboats.

In addition, the Navy is dispatching a four-craft fleet of speedboats manned by its elite SEAL -- sea, air, land -- unconventional warfare teams. Pentagon sources say the speedboats will be supplemented by an Army special forces unit using four surveillance and counterattack helicopters that can be launched from ships in the gulf.

But it has taken several weeks and the mining incident to steer military officials toward trying to outguess the enemy rather than simply reacting, according to Pentagon sources.

First came the decision to increase the size of the conventional shipping force in the gulf from five to nine vessels, most of them frigates, destroyers or cruisers better equipped to handle tasks of fending off potential attackers.

U.S. officials considered the defenses adequate to handle Iran's aging fleet of ships and aircraft, many of which were provided by the United States before the fall of Shah Mohammad Reza Pahlavi in 1979. Iraq claims it has destroyed many Iranian warships, and "naval defectors have said that Navy strength is very low," according to the 1987 edition of the World Defense Almanac. Most recent information indicates the Iranians now have one British-type and two U.S.-type destroyers and four frigates.

The Iranian Air Force has been particularly hard hit by a shortage of spare parts for its American-built F14 fighter planes. Intelligence sources estimate that five to 15 of the fighters are operational. The almanac also reports that Iran has received an undetermined number of fighter planes from China.

Defense experts note, however, that the Iranians frequently deploy aircraft in greater numbers than many western analysts anticipate.

But the potential threat of Iran's aging naval and air forces has been overshadowed by new concerns over its possible deployment of Chinese-made surface-to-ship Silkworm missiles from strategic sites along the Strait of Hormuz. The number of Iranian antiship missiles, which have a range of 50 to 60 miles, has increased from about 20 earlier this year to an estimated 50 or more, with reported new shipments from China, according to U.S. intelligence information.

In response to those threats, the Navy moved the aircraft carrier USS Constellation and its five support vessels into the northern Arabian Sea where its aircraft could be used as protection for shipping interests in the gulf.

One of the most valuable weapons U.S. forces have been using to counter the Silkworm threat is the AE6B Prowler, an airplane loaded with electronic surveillance and control systems with 12 high-power jamming channels.

Within the past year, the Prowler has been equipped not only to jam radars used by Silkworm crews searching for targets but has also been armed with the HARM (high speed antiradiation) missile designed to destroy the electronic emitters that direct missile attacks. Military officials have not publicly discussed the aircraft's ability to counter the threats of Silkworm missiles.

"The Prowler has significantly reduced the threat of the Silkworms," said one Pentagon official. Another official cautioned, however, "It's not a cure-all. You have to have good intelligence and everything has to work."

A Navy Prowler from the Constellation picked up radar emissions near Silkworm launchers during the last hours of the reflagged tanker Gas Prince's trip through the Strait of Hormuz last Monday, Pentagon sources said. After a frenzied three or four hours, military intelligence officials determined the radar signals had been a "false alarm" and no missiles were loaded into the mobile launchers spotted earlier.

But it was not the Silkworm or the Iranian naval and air forces that marred the first escorting effort. The Bridgeton struck a submerged mine that U.S. officials believe was planted by the Iranians in the shipping channel just hours before the convoy moved through the area.

Early this week, eight Sea Stallion mine-sweeping helicopters are scheduled to arrive in the gulf on the helicopter carrier USS Guadalcanal, which left 700 Marines and their equipment on the tiny Indian Ocean island of Diego Garcia last week to make room for the new mine-sweeping forces.

Pentagon officials said the Sea Stallions will work in shifts, trolling the waters ahead of the reflagged Kuwaiti tankers and their U.S. warships. Officials note, however, that the helicopters usually are not used at night because of safety concerns, and the convoys probably will have to rely on ship mine sweepers after dark. A transport ship carrying four small mine-sweeping boats from Charleston, S.C., is not scheduled to arrive in the gulf for more than three weeks.

Late last week, the Iranians unveiled yet another threat -- a small 20-to-30-foot-long submarine. Pentagon officials said it could be used in the shallow waters of the gulf to lay mines or move small groups of special forces to gulf shores.

"You've got to worry about it now," said one Pentagon official. "It's out there." Other officials familiar with the small submarine say they view it as one more effort to divert the U.S. military's attention from more imminent threats.