During a briefing last month at the Philippine presidential palace that lasted less than an hour, President Gloria Macapagal Arroyo and top aides were told by senior U.S. officials of a Pentagon plan to send U.S. Special Forces troops into the southern Philippines to attack the Abu Sayyaf, a militant Islamic rebel group.

According to a participant and other officials familiar with the plan, Arroyo was presented with a risky operation in which U.S. forces would enter hostile territory. By contrast, in a successful six-month U.S.-Philippine counter-terrorism mission last year in the southern part of the country, U.S. forces were deployed most of the time far from the front lines.

At the Feb. 4 briefing, the officials said, both sides believed they had an agreement. Not least important was language: The Filipinos knew that because of the risks soldiers would face, the United States had to call the undertaking an "operation." The Americans knew that because of political and constitutional constraints, the Filipinos had to cast it as "training" and an "exercise," the officials recalled.

But last week, the plan was put on hold, raising questions about how it was put together in the first place. Some details about the process remain unclear, but in interviews here and in Washington, officials described a series of missed signals and apparent misunderstandings that led to an embarrassing public reversal. The Pentagon announced Friday that the plan was being frozen.

Some sources questioned whether Philippine officials had given unwarranted assurances to U.S. military planners, allowing them to proceed when they should have been more cautious. Others, including some U.S. officials, asserted that the Pentagon failed to grasp the political and cultural sensitivities in the Philippines, a former U.S. colony in which nationalist sentiment led to the closure of two U.S. military bases a decade ago.

What is clear is that U.S. officials believed the Philippine government supported the plan they were designing. Philippine officials believed that as long as the plan could be considered a training exercise, they would be on safe legal and political ground. The semantics used to describe the mission became very important, but neither side anticipated that the decision to cast the mission in two different ways would eventually backfire.

"It was understood that each country would not use exactly the same language to describe this," said a U.S. official with knowledge of the briefing. That agreement was made at the meeting and later reflected in a Pentagon "execute" order that specified that the operation would be described as an "Armed Forces of the Philippines-led, U.S.-supported operation" that would include training, exercise and operational components, he said.

"Manila was going to emphasize the training and exercise aspect," the official said. "It's not that the word 'operation' was verboten. It's just that it wouldn't feature as prominently in how it would be described there."

On Feb. 17, Arroyo's spokesman, Ignacio Bunye, announced an "exercise" focused on "increasing our ability to protect ourselves against terrorist threats."

Three days later, a Pentagon spokesman in Washington, authorized to brief reporters and cognizant of the need to be frank about the mission's nature, announced the plan in different terms. The United States would send 350 Special Forces troops to the southern island of Jolo, an Abu Sayyaf stronghold. They would be joined by 2,700 support troops. "The intent," the spokesman said, "is for U.S. troops to actively participate" in the operation. He made clear that combat would be involved.

Reaction was swift and fierce. Lawmakers in Manila denounced the plan as an affront to Philippine sovereignty. Leaders in the Muslim-dominated south warned of an anti-American backlash. Philippine government officials derided the Pentagon statements as unauthorized "leaks."

"On the technical level, probably, both sides understood each other," said Alex Magno, a Philippine political analyst. "It's on the public diplomacy level where you had problems."

In casting their respective explanations, U.S. officials needed to prepare the American public for what would likely be a dangerous operation, while their Filipino counterparts had to affirm that the involvement of the Americans would not violate a constitutional prohibition against combat operations by foreign forces in the Philippines. But the differing public explanations did not go over well.

Defense officials in Washington stood by the phrasing used by the spokesman, who they said adhered to talking points carefully crafted by the Pentagon's policy branch.

"He wasn't freelancing," a senior defense official said. "If we're going to get involved in this way, we have to make sure the public and the Congress know exactly what we're doing."

The plan was modeled after last year's exercise on Basilan island, about 70 miles northeast of Jolo (pronounced Holo). With the aid of U.S. advisers, Philippine soldiers and marines largely rid Basilan of the Abu Sayyaf, though two rebel factions fled to Jolo. The mission, called Balikatan, or shoulder-to-shoulder, drew praise from residents not only because it vanquished the rebels but also because U.S. troops built roads and wells and delivered medicine.

The Philippine government asked the U.S. officials last year if they could replicate the exercise on Jolo. The United States wanted to help an ally in the war on terrorism. An explosion linked to the Abu Sayyaf last October that killed a U.S. soldier on Mindanao, the main southern island, fueled U.S. resolve.

Last November, officials said, Defense Secretary Donald H. Rumsfeld directed the military's Pacific Command and Joint Staff to design a mission like the one carried out on Basilan.

By January, the plan was taking shape. A budget was drawn up, troop units identified, rough time frames set. A provision was included for sending U.S. Special Forces on combat patrol. A similar provision had been made for Basilan -- but it wasn't activated until the final weeks of the mission because of its political sensitivity.

By early February, officials from the Pacific Command, the Pentagon and the State Department were preparing for their trip to Manila. A cable spelling out the government's proposal was sent to Manila before their arrival. The understanding, U.S. officials said, was that the mission would be similar to Balikatan, but would involve U.S. troops to advise at the platoon level of 20 to 30 soldiers. "It clearly was understood that this meant a higher degree of exposure to hostile fire" for U.S. troops, an American official said.

Also, a U.S. Marine contingent would be deployed in the Sulu Sea: Because Jolo is farther south than Basilan, planners decided that they could not rely on a backup force in Mindanao as they had last year and needed to place the Marines on amphibious assault ships.

At the presidential palace, according to participants, Arroyo and her aides liked the concept, as long as it mirrored the Basilan exercise, with Philippine officers in command and U.S. soldiers assisting. "Maybe this was their perception that this was a combat operation, but for our part it was clear this remains assistance," said a Philippine government official who was at the briefing.

"I think the problem here is that the Pentagon planned it on the assumption that it would be okay the way they presented it," a Philippine defense official said. "They have to redo it if they want it to conform with the requirements of the Philippines. If they want to redo it, they have to find a new budget, or cancel it, or call it a new thing."

This year's meltdown resembles an impasse last year, when disagreements over the U.S. role and how to describe it threatened to stall the Basilan exercise. A compromise was reached that allowed each side to call it what it wanted. This time, with the stakes higher, the incongruence of the language used was magnified.

Sen. Rodolfo Biazon said he is hoping a congressional hearing will help answer questions such as whether Philippine officials, including Defense Secretary Angelo Reyes, "might have given the Americans the impression that they would be allowed to participate in actual combat operations." The assumption, he said, is that the Pentagon could not have made its Feb. 20 announcement without reassurances from Philippine officials.

Rumsfeld expressed optimism that there will be activity in the Philippines this year resembling the Balikatan mission. "I'm absolutely certain of that," he said, adding that such activity would not violate the Philippine constitution.

Graham reported from Washington.

Philippine troops and U.S. Marines rappel down from a helicopter during a joint exercise about 25 miles southwest of Manila. A plan for another joint mission elsewhere in the Philippines is on hold.