Sean Bell and his two companions didn’t have a gun between them the night they faced a fusillade of 50 police bullets outside a strip club in Queens. As the trial of the two detectives charged with Bell’s 2006 death opened yesterday in Kew Gardens, their lawyers told Judge Arthur Cooperman, who is trying the case, to put that fact out of mind.

That the men turned out to be unarmed, a defense lawyer, James Culleton, said, is “hindsight wisdom.”

“Monday-morning quarterbacking has no place in a court of law,” he added.

Defense lawyers for detectives Michael Oliver and Gescard Isnora mounted more than a defense against the charges of manslaughter yesterday. They also defended the decision the detectives made at the time, which was to open fire in the face of a threat that turned out, in hindsight, to be only perceived. The detectives’ decision led to the death of Bell, 23, on the morning of his wedding day, and the wounding of Bell’s two friends in his Nissan Altima.

The defense lawyers set as their task to prove that detectives Oliver and Isnora had reason to believe that shortly after 4 a.m. on November 25, 2006, one of Bell’s two friends in that car, Joseph Guzman, was armed.

At the disposal of the defense are little more than snippets of a conversation that Detective Isnora, working undercover, had overheard in the parking lot of the strip club, Club Kalua, at closing time. Mr. Guzman had pledged to get a “gat” — slang for a gun — to help Bell settle an argument with a stranger who was the boyfriend of a stripper, Detective Isnora’s defense attorney, Anthony Ricco, said. That boyfriend will testify, defense lawyers said.

The shooting began moments later when Detective Isnora confronted Mr. Guzman, Bell, and a third member of their party, Trent Benefield, as the three sat in Bell’s car.

“Something extraordinary happened that night,” Mr. Culleton, who represents Detective Oliver, said. “Five experienced officers believed they were being fired upon.”

The five members of an anti-vice squad focused on nightclubs all fired their guns. Two were not charged. Another detective, Marc Cooper, who is being tried together with detectives Oliver and Isnora, faces a lesser charge of reckless endangerment.

The trial, expected to last about two months, will focus primarily on Detective Isnora, who spent most of night undercover inside the strip club looking for evidence of drugs or prostitution. Detective Oliver was outside at the wheel of a police van without any firsthand knowledge of the men he would soon open fire on. As defense lawyers described each man’s actions, it became clear that the defenses of the two detectives are not entirely complementary.

Detective Oliver’s decisions that night, his lawyer suggested, followed directly from the assessments that Detective Isnora made.

So, when Detective Oliver came on the scene to see Detective Isnora, gun raised, shield around his neck, standing over the parked Altima and yelling the word “gun,” Detective Oliver “had every right to rely on Detective Isnora,” the lawyer, Mr. Culleton, said.

Of the perhaps 100 patrons of Club Kalua that night, the bachelor party for Sean Bell was responsible for less than a dozen. The party was organized that night after the groomsman-to-be dropped his fiance, Nicole Paultre, off at her bridal shower. The party stretched on for five hours at the strip club, where a sign at the entrance tells patrons to order a drink every half hour.

It was the sort of place, Detective Isnora’s attorney, Mr. Ricco, said, that “attracts a negative element” and is frequented by people wanting “to feel a twisted sense of prowess.”

Mr. Ricco described Bell as leaving the club in an angry mood, drunk enough that the first thing he did, after urinating, was to try to pick a fight with a man who was waiting outside to pick up his girlfriend, a stripper at the club.

“He put his marriage on the back burner,” Mr. Ricco said. “He decided to take on the man with the SUV.”

Mr. Ricco said the idle threat he alleges Bell to have made — “I will shoot you” — sounded real to any detective within earshot.

The lead prosecutor, Charles Testagrossa, described what Bell likely saw in his last moments.

Sitting in the car, Bell would likely not have even known that Detective Isnora, approaching the car with gun raised, was law enforcement, Mr. Testagrossa said.

“He never identified himself as a police officer” Mr. Testagrossa said. The detective’s shield around Detective Isnora’s neck, the prosecutor said, would not be visible if the man’s chin was lowered or his shoulder turned.

Bell hit the gas, trying to maneuver his car away, swiping Detective Isnora and hitting the van carrying Detective Oliver.

Of the gunfire that ensued, Mr. Testagrossa said that had Detective Oliver, who fired 31 shots, “paused to reassess, he would have learned that no gunfire was coming from the car.”