In one sign of the ongoing war on workers in the United States, employers are increasingly resorting to one the most brutal tactics in the anti-union playbook: the lockout.

Companies as diverse as Caterpillar, American Crystal Sugar, Sotheby’s, Consolidated Edison and the New York City Opera have all locked out their workers during contract disputes over the last several years. The National Hockey League, the National Football League and the National Basketball Association all have done the same.

Lockouts are a show of raw power that delivers a simple message from employers to workers: You work only if we let you.

“What we’ve seen in recent years is the contract expires and the employer locks out the workers even though they are willing to keep working,” said longtime labor lawyer and negotiator Joe Burns, who has written extensively on union strategy. “Employers have gotten very aggressive and are using the lockout to enforce their demands.”

Strikes are for workers what lockouts are for employers. But the number of strikes has fallen to one-sixth of what it was 20 years ago. Employer lockouts now represent a growing percentage of the country’s work stoppages, up from 4.4 percent in 1990 to 11.3 percent in 2011, when there were at least 17 lockouts.

“Employers were much more likely to lock out workers in 2011 than in any other year since we started keeping track of work stoppages two decades ago,” Bloomberg BNA’s Robert Combs wrote in a January 2012 blog post.

Over the past decade, there have been about 10 strikes of 1,000 or more workers each year. That is a big change from the 1950s when in any given year there were on average 350 strikes involving more than 1,000 workers.

Burns, the labor lawyer, attributes the increasing use of the lockout and declining use of the strike to employers’ increased influence in Congress and on the courts, which has led to labor law changes unfavorable to workers.

Indeed, lockouts were only legalized in 1957, as a result of a Supreme Court decision, and for many years employers could use lockouts only as a “defensive” measure, if the employer believed a strike was imminent. However, since the 1960s, Burns said, employers have been allowed to use lockouts as an offensive measure as well.

“It’s really an illegitimate tactic,” he said. “All the workers are trying to do is bargain, and then the employer responds by saying he’s going to take away their employment.”

The consequences for workers can be disastrous. While some lockouts last only for a matter of days or weeks, others drag on for months or even years, in many cases doing irreparable harm to workers’ lives and livelihoods.

That destructive power of the lockout is on display today at a Kellogg plant in Memphis, where the company has locked 226 union workers out of their jobs for more than seven months.

Kevin Bradshaw, the president of the workers’ union — a local of the Bakery, Confectionery, Tobacco Workers and Grain Millers International Union — described the devastating financial toll. Some workers have lost their homes, and many have lost their cars, he said. Others have struggled to pay medical bills for themselves and their families since the company cut workers’ health insurance the first day of the lockout. Bradshaw himself has struggled to find health insurance for his four children and has all but depleted their college savings.

He emphasized the emotional toll.

“It’s heartbreaking,” Bradshaw said. “It feels like you’ve been violated, past the point of being able to recover. We have people who have been here 54 years, people who have given their whole lives to the company. They’ve never had the company do this to them before, not being able to work while we try to sort things out.”

“This company is not the company we used to work for,” Bradshaw concluded. “They don’t care about hardworking Americans because they just want more profit.”

With the growing number of lockouts, many U.S. workers are coming to the same realization about their employers.