NAJAF, Iraq — In a shabby but spotless living room in the holy city of Najaf, a top deputy of Shiite leader Moqtada al-Sadr quietly sketched out his vision of the Iraq to come, after the Americans withdraw.

First, “there will be a civil war,” the aide, Mustafa Yaqoubi, said, as his three young children wandered in and out of the room. The rising violence and rivalries under the American occupation make a shaking-out all but inevitable once foreign forces go, Mr. Yaqoubi said. “I expect it.”

“No matter the number of people who would lose their lives, it is better than now,” he added. “It would be better than the Americans staying.”

When the tumult ends, Mr. Sadr’s aide said, Iraq’s Shiite majority finally will claim its due, long resisted by the Americans — freedom to usher in a Shiite religious government that Mr. Yaqoubi said would be moderate and perhaps comparable in some ways to Iran’s. The bespectacled, bearded cleric’s mild tone buffered his talk of the blood that would have to be spilled to achieve this goal. No matter when the Americans withdraw, “the first year of transition, it will be worse,” Mr. Yaqoubi warned. “After that, it will gradually improve.”

Mr. Yaqoubi speaks as one of two or three long-time intimates of Mr. Sadr, the young heir of a revered Shiite clerical family. Mr. Sadr’s rough-edged street movement of poor, largely uneducated Shiites has burgeoned into one of the strongest political and armed forces in Iraq.

When Mr. Sadr’s father, Grand Ayatollah Mohammed Sadiq al-Sadr, was shot to death with two of his sons in 1999, in an assassination blamed on Saddam Hussein, Mr. Yaqoubi helped Mr. Sadr, then only 25, keep the family’s mosque-based network alive, despite unrelenting pressure from Saddam’s intelligence services.

Mr. Yaqoubi’s arrest by American-led coalition troops, on charges in the brutal stabbing death of a rival of Mr. Sadr, ignited full-scale street battles between Mr. Sadr’s followers and America’s forces in April 2004.

Freed in August 2005 after 16 months in prison, Mr. Yaqoubi helped preside over a remarkable political transformation that culminated with elections last year that put Mr. Sadr in charge of the largest individual bloc in parliament.

While Mr. Sadr and his aides, including Mr. Yaqoubi, stayed in Iraq throughout the darkest years of Saddam’s rule, others among Iraq’s current leaders went into exile in London, Detroit, or Tehran, Iran, returning here only after Saddam’s overthrow. Leaders of the other main Shiite religious parties were quick to make accommodation with the American-led occupying forces. Overnight, many of them adopted a life of second-hand splendor in the former palaces and villas of Saddam’s regime.

Abdul Aziz al-Hakim, for example, who leads the Shiite religious party that most nearly matches Mr. Sadr’s movement in strength, moved into the marbled villa of former Saddam foreign minister Tariq Aziz, who is now in American military custody. Mr. Sadr and his aides, in contrast, make their homes in the Shiite neighborhoods of the capital and the Shiite holy cities of the south. Mr. Sadr lives in a gleaming, whitewashed concrete mansion behind high walls in Najaf. His top aides there have more modest houses.

Mr. Yaqoubi is considered an intellectual, in the vanguard of Mr. Sadr’s Shiite movement. In an afternoon interview, he outlined his views of an organization that is scarcely known to Americans. His children occasionally came in to interrupt, putting a hand on his knee to whisper a message from the women out of sight in the back of the house.

Despite their ascendancy now, Mr. Yaqoubi said, Iraq’s Shiites owe no gratitude to the Americans. “The Americans are not saving us from Saddam for the sake of the Iraqi people,” he said. “They gave Saddam clearance in the 1990s to strike at the Shia people. It was in their own interest to get rid of Saddam.”

According to Mr. Yaqoubi, the Americans brought the armed resistance on themselves by staying after the invasion and by ignoring Iraqi protests. For example, he said, tens of thousands rallied this summer in Baghdad’s Shiite neighborhood of Sadr City to protest the Israeli invasion of Lebanon, but the Americans ignored them. “It was the largest rally in the world. But with them, it’s useless,” he said, referring to American officials.”No one ever reacts. No one responds to these protests.”

Ordinary Americans, on some level, must understand the resistance to the foreign forces, he said.

“We believe the American people are not coming from Mars.They see on their televisions how it is here,” Mr. Yaqoubi said. “They have the same mentality we have. We believe that if the Americans were occupied by another country they would do the same as we are, or even more.”

Mr. Yaqoubi said the American failure to meet even the simplest security needs of Iraq was to blame for much of the current instability. As a result, he said, “When the Americans pull out, there will be a civil war. They are using that now as an excuse for staying.”

Mr. Sadr’s deputy spoke confidently and simply of which faction would emerge the winner. “I don’t want to use this expression, but you have an expression,” he said.”‘Survival of the fittest; the strongest survive’?”

He added, “If there may be other forces to use their strengths, I don’t think they have the capability to match us.”