How did the situation in Iraq change so quickly from rapid military success to endless insurgency? “The Lost Year in Iraq” on “Frontline” (8 p.m., PBS, check local listings) looks at the key decisions made by American ambassador L. Paul Bremer III and others in 2003 that radically changed the relationship between Americans and the people they had come to liberate.

The documentary contains more than 30 interviews with Bremer and military personnel, CIA officers, reporters and authors, including Thomas E. Ricks, author of “Fiasco: The American Military Adventure in Iraq” (Penguin, 2006) and Rajiv Chandrasekaran, author of “Imperial Life in the Emerald City: Inside Iraq’s Green Zone” (Knopf, 2006).

Bremer’s first edict, purging former members of Saddam Hussein’s Baathist party from the government, affected everyone from key ministers to grade-school teachers. For many, particularly Sunnis, the Americans were now the enemy.

But Bremer’s decision to disband the Iraqi army had more immediate and dire consequences. Many of these defeated soldiers had been looking forward to joining with the Americans to provide stability as police officers. They were also looking forward to paychecks.

According to counterinsurgency expert Col. T.X. Hammes, Bremer’s order changed everything. “Now you have a couple hundred thousand people who are armed … who have no future, and who have a reason to be angry with you.” Another expert, Franklin C. Miller, recalls, “We believed it would be dangerous to put 300,000 men on the street with guns, without jobs.”

Almost immediately, the insurgency changed from sporadic looting to military-style attacks that continue to this day.

Bremer deserves credit for appearing on “Frontline,” but his contention that he and others were making a lot of decisions at a very busy time is less than convincing.

Tonight’s other highlights

¢ An unexpected loss puts the coach on the spot on “Friday Night Lights” (7 p.m., NBC).

¢ Lorelai has to come up with bail money again on “Gilmore Girls” (7 p.m., CW).

¢ The squad must airlift an exiled tyrant from an American hospital on “The Unit” (8 p.m., CBS).

¢ Two very different brothers die under similarly suspicious circumstances on “Law & Order: Criminal Intent” (8 p.m., NBC).

¢ A repeat offender may have left a clue on “Law & Order: Special Victims Unit” (9 p.m., NBC).

¢ Alan Shore (James Spader) tangles with Scientology on “Boston Legal” (9 p.m., ABC).