He first tried to get to Iraq in April 2003, when U.S. troops established control over the country and jihad became a place on a map.

"I wanted to come and fight for Islam," said Abu Thar, who started the journey from the capital city of his native country, Yemen, across the Arabian Peninsula. "I met a Jordanian merchant who provided me with tickets to Syria and a hundred dollars.

"He even drove me to the airport himself."

Abu Thar arrived at the airport in Sanaa, Yemen's capital, with a group of other Yemeni students, a flock of would-be jihadis forming a neat line at the immigration counter. Abu Thar was wearing a traditional Arab robe and a small turban.

"And when the police asked me why I was going to Damascus, I said, 'To work.' They asked me what kind of work. I said, 'To work for the salvation of my soul.' And they sent me back."

A thin young man with an ascetic manner and a gentle voice, Abu Thar fingered the fabric of his cheap cotton trousers. By his reckoning, the Western clothes were what finally got him started on the smugglers' road to Iraq, in time for the showdown in Fallujah.

"This time," he said, "I learned the lesson and bought these."

If foreign fighters are the primary stated reason that 10,000 U.S. troops this week commenced the largest combat operation since the fall of Baghdad, the journey of Abu Thar sheds rare light on their presence in Fallujah. Arab fighters poured in when Marines first laid siege to the city in April. At the time, many Fallujans welcomed the foreign fighters as reinforcements against an occupation force that many felt was punishing an entire city for the actions of the few who had mutilated the bodies of American contractors days before.

But in the six months that followed, by many accounts, a coolness developed between hosts and guests. The Arabs were blamed for beheadings, car bombs that killed civilians and for imposing their strict notions of faith on a local population with traditions of its own.

In the end, the stubborn presence of foreign fighters scotched efforts to return control of Fallujah to Iraq's interim government. By Monday night, when U.S. tanks rolled into a city largely emptied of civilians, American commanders estimated that Arabs from countries other than Iraq accounted for at least 20 percent of the 3,000 or so fighters who remained.

This is the story of one.

It was related to a reporter Saturday, two days before the start of fighting, in a soft voice sometimes drowned out by the percussion of artillery shells. He stood beside a makeshift bunker, in an Adidas jacket and athletic shoes. The specifics of Abu Thar's story could not be immediately confirmed. But its broad outline -- of a kind of underground railroad channeling young Arab fighters into Iraq -- is consistent with other accounts, and the declarations of U.S. and Iraqi officials.

The manner of the individual relating them, moreover, was sincere in the extreme. Abu Thar, a nom de guerre, told the recent story of his life with the evident earnestness that moved his fellow fighters in Fallujah to elect him to lead the group in prayers, an honor normally reserved for the commander.

He said that the day after he was turned away at the airport, he returned to his job driving a minibus taxi in Yemen's capital. His passion was the study of Islamic law, or sharia. But after Sept. 11, 2001, Yemen's government cracked down on foreign support to the religious university where he had studied for six years, and he no longer received the $50 monthly stipend on which he lived. So he drove a cab.

A year passed. Abu Thar turned 30, and might never have tried to reach Iraq again but for the photographs that emerged of U.S. military police abusing prisoners at Abu Ghraib prison. Seeing the photos, his wife, also a religious student, urged him to leave everything and go to Iraq to fight jihad. She was pregnant with their sixth child.

"She told me, 'If they are doing this to the men, imagine what is happening to the women now,' " Abu Thar recalled. " 'Imagine your sisters and I being raped by the infidel American pigs.' "

He said he spent the night crying, tormented that he did not persevere earlier. In the morning he started making the rounds of friends, borrowing money to travel. From the Jordanian man, he got airline tickets to Syria. From his university, run by a senior Yemeni cleric, he got the name of a man in Aleppo, a city in northern Syria, who would arrange for him to be smuggled across the desert border into Iraq.

He said he told no one he was going. "I just told my wife. I borrowed a car from a friend, and we went out to do some shopping. She bought me two trousers and a shirt. We went then to my father's house. I told my mother, 'Forgive me if I had done anything wrong.' She said, 'Why?' I told her, 'Nothing, I just want forgiveness from you and Dad.'

"She asked me if I was going to Baghdad. I said no. She hugged me and cried."

At the memory, tears formed in Abu Thar's eyes. He wiped them with his checkered headdress and blamed the rain.

Back at his home, he had a final dinner with his wife and children, who went to bed without being told their father was leaving. "My favorite daughter came and sat in my lap and slept there. She opened her eyes and said, 'Daddy, I love you.' "

He was weeping openly now, a thin man with a thin beard under a ragged tree in a courtyard in Fallujah. "You know these memories are the work of the devil trying to soften my heart and bring me back home," he said.

He rejected going home with a passion. When a visitor told him, "We will come and see you and your family in Yemen," the anger in his reply contorted his usually smooth features. "The only place I am going from here," he snapped, "is heaven."

By the time he reached Damascus, the word from jihadi networks was that the Syrians had tightened security on the border with Iraq. For weeks, Abu Thar waited, moving from safe house to safe house -- cheap hotel rooms in Damascus, Aleppo, Hams, sometimes a mosque, or a mattress in a room above a religious school. In each place, he said, he found himself quarters with another dozen young men making their way to Iraq.

Eventually, he reached Aleppo, near the border.

There, he said, he met a young cleric who promised to help. He spent two weeks waiting in a small house filled with other jihadis. Each had a coordinator back home, usually the leader of a mosque or another prominent person who had vouched for him. Abu Thar, arriving on his own, was at first considered suspicious. That was "until they called my master in the religious school in Yemen," he said.

One night seven weeks ago, he was taken to a village on the Syrian side of the border. The border police were paid to look the other way, he said.

"They came and said, 'We are crossing today.' It was a very scary journey. We had to lay still in the desert if we heard American helicopters.

"We spent two nights on the border in a village, then we were taken to another village to be given military training. Most of the brothers with me have never used a weapon in their life. I knew how to use an AK-47.

"After a few days they came and said, 'We need fighters to go to Hit,' " an Iraqi city on the Euphrates River halfway to Baghdad.

In Hit, he found himself in a trench beside other Arab fighters, believing his dream of martyrdom was within reach. But a cease-fire was called, and the Arabs were ushered into a minibus and smuggled east. Of the two vehicles serving as escorts on the night drive, he said, one was a police car.

In Fallujah, Abu Thar was assigned to a group called Monotheism and Jihad. The group is headed by Abu Musab Zarqawi, a Jordanian who has asserted responsibility for many of the most extreme terrorist strikes in Iraq, and who last month allied his group with al Qaeda.

Abu Thar was assigned to a platoon with 11 other men, only three of them Iraqis. They were stationed in a safe house in the Jolan neighborhood of Fallujah's northwest corner, where Marines later entered behind M1-A1 Abrams tanks.

Two days before the battle, Abu Thar read aloud from a small Koran in a half-lit room with walls bare but for a picture of Mecca. There was no furniture, only a prayer mat twisted at an angle to face south toward Mecca. A Kalashnikov assault rifle and an ammunition pouch were laid against the wall.

When he finished reading, he held his hands high and prayed: "Oh God, you who made the prophet victorious in his wars against the infidels, make us victorious in our war against America.

"Oh God, defeat America and its allies everywhere.

"Oh God, make us worthy of your religion."

He spoke with the calm air of an ascetic. And, indeed, of the 12 fighters in the house, Abu Thar seemed the least martial by far. Some of the men spoke as if they loved death, but he spoke of dying on the battlefield as something more like devotion. To him, martyrdom was the purest way to worship God.

"When I was in Syria, I bought seven copies of this,'' he said, pulling a pocket-size copy of the Koran from his jacket. "I wrote the name of my wife and my five children on each and left the seventh empty."

He said he did not want to impose a name on the child his wife was carrying when he left. But just before he crossed the border from Syria, she called and told him she had given the child a name: Shahid. It means martyr.

Abu Thar rested Sunday. He didn't plan to return to Yemen, where his wife and six children live. "The only place I am going from here," he said, "is heaven."