BROOKLYN -- This is a report from a battlefield, not in the Middle East but one of the many in our midst.

To get to the 75th Precinct from police headquarters in lower Manhattan, you take the Brooklyn Bridge, the engineering marvel of the 19th century. It was built in the full flush of American optimism after the guns of the Civil War fell silent, back when Brooklyn was a separate city, the nation's third largest, and building bridges symbolized the knitting-together of a nation that had been bloodily divided.

Today the trip takes you into the most shocking division in America, not the perennial division between rich and poor, but between the two Americas where gunfire is, and is not, part of the ambience of normal life. The 75th is beyond the imagining of most Americans, who haven't the foggiest idea of the dangers and affronts that many of their fellow citizens experience daily.

Even Pat Carroll, 49, almost seems to have a hard time believing it, and he has lived in the city all his life and today is the commanding officer of the 75th. He sits behind one of those government-issue metal desks. He faces, across his cramped and cluttered office, a map of a sort no police officer faced in 1965 and no layman can easily fathom. It is covered with green and red circles. They denote places in his precinct -- reds are indoors, greens outdoors -- where drugs are routinely sold.

The layman's instinct is: What are you waiting for? Go get 'em. But the layman has not had the bailing-the-ocean fatigue that comes from sending minor drug dealers into the criminal-justice system only to see them replaced on the street corners even before the system spews them back onto the streets.

"Arrests aren't the answer," Carroll says, "but we can't not do that. It gives the community heart." But the community, he says, is not convinced when better arrest statistics are announced to a background staccato of gunfire.

There are in the 75th about 32 arrests a day, two-thirds of them drug-related. Arrest numbers could be tripled, given enough officers, courts and prison cells. But in a city that is financially on the rocks and emotionally on the ropes, police work requires an endless series of cold judgments, apportioning shrinking human resources to an expanding problem.

The 75th ranks second in the city in 9ll radio runs, and an officer is injured almost every day, often in scuffles in domestic disputes. But such disputes are hardly the worst of the pandemic violence.

"When I was growing up," Carroll recalls, "if there was a fight, you might get punched in the nose, or someone might swing a garrison belt. Nowadays someone says 'He dis'd me' " -- showed disrespect -- "and guns come out." Today's weapons of choice are semiautomatics with magazines holding from nine to 25 rounds. "Twenty-five years ago," he says, "you made a gun collar {arrest}, it was {a} big to-do." In 1990, there were 1,194 gun arrests in the 75th.

When today's 49-year-old policeman was a rookie in the 1960s, robbery was the crime that defined a neighborhood at risk. Nowadays the crime is drug dealing, with accompanying gunfire.

Of course, robbery is rampant because drug habits must be fed. (If you must steal $l0 worth of property to raise $l from a fence, then a $l00-a-day habit requires $l,000 worth of stolen stuff.) Much of the gunfire is connected with routine practices of the drug trade -- claiming territory, punishing people who do not fulfill contracts.

Carroll, whose son and daughter are cops, has a master's degree in urban affairs and a quarter of a century of on-the-street education, all of which tells him this: Police will be overwhelmed until the rest of government gets on with its jobs of enacting gun controls, providing drug treatment and treating the seedbed of most crime, the dysfunctional families that send forth violent young men.

Furthermore, Carroll is convinced, as so many cops are, that society's forces for order are no match for today's popular culture. You do not talk long with cops before they mention movies that are desensitizing young people by glorifying casual brutality.

You say cops should leave sociology to the social scientists? Cops lead lives rich in instructive anecdotes, enough anecdotes to justify generalizations. Cops know that business is booming here for companies offering armed escorts and selling bulletproof vests for children.

There may be a New World Order being built beyond our shores. At home, there is an accelerating failure to (in the Constitution's words) ensure domestic tranquillity.