Brian Poulsen fought on the front lines of America's war against illegal immigration for three decades. He patrolled the border near San Diego on horseback, grew a ragged beard to disguise himself as a human smuggler, arrested and deported thousands of illegal immigrants, and tracked down fake document vendors at local flea markets.

His job description was clear: Do everything you can to stop illegal immigrants from coming and kick them out if they get here.

But Poulsen and other agents have discovered it's not easy to enforce immigration laws — especially those that target employers.

Aggressive crackdowns can backfire if they hurt business. A sweep of Midwestern meatpacking plants in the late 1990s, for example, prompted outrage from business and civic leaders. Immigration officials have learned to tread lightly.

Poulsen, who retired this year as the top immigration enforcement official in the central San Joaquin Valley, tried to strike a balance between stopping illegal immigration and protecting farmers' interests. His office rarely conducted audits, never issued a fine and avoided messy, high-profile raids that would permanently shut down a business and separate families.

"There's a little bit of a tightrope. I understand where the farmers are coming from," said Poulsen, who grew up harvesting potatoes in Idaho. "You don't want to see people go out of business, but at the same time, we're sworn to do a job and can't look the other way."

Things may be changing: The Obama administration has stepped up the pace of audits, which are less likely to spark a backlash than workplace raids. For example, of 16 audits conducted in the central San Joaquin Valley over the past eight years, 11 have come since late 2008.

Some agriculture leaders in the Valley are worried about the audits, which can hurt businesses by making them fire all their illegal workers.

But government figures show that the new effort is tame compared to the early 1990s, when immigration officials fined about 900 companies a year and audited thousands. This year, they fined 237.

Many experts say aggressively cracking down on employers will rid the nation of illegal immigrants. Because the vast majority of them come here to work, America would be a much less appealing destination without job opportunities.

But advocates for stricter enforcement say there is little political appetite to sever the co-dependent relationship between businesses and illegal immigrants.

And aggressive enforcement, agents have learned, can backfire.

In the late 1990s, in response to calls for tougher enforcement, the U.S. Immigration and Naturalization Service — which later became Immigration and Customs Enforcement — blanketed Midwestern meat-packing companies with audits that identified thousands of employees suspected of being illegal immigrants.

Frightened workers left in droves, slowing slaughter-line speeds to a crawl and hurting farmers who relied on the meat-packing businesses.

"All hell broke loose," said Mark Reed, a former top INS official who was in charge of the crackdown, dubbed Operation Vanguard. "All of a sudden, these communities that wanted these people out of there realized that they needed them. They didn't realize that the kids who played on their soccer teams and the people they went to church with were going to go. They didn't realize that they were the center of their economy."

Nebraska's governor and members of Congress intervened. Operation Vanguard — which was scheduled to expand to the rest of the country — was nipped in the bud.