In early 1970, National Guardsmen were spotted walking from door to door in neighborhoods throughout the United States. They weren’t conducting a military operation or helping clean up after a natural disaster—they were delivering the mail in the midst of a postal strike that almost brought the United States to a halt.

The eight-day strike of some 150,000 letter carriers in 30 cities took the nation by surprise, but for many in the U.S. Postal Department (forerunner of today’s USPS) it was a long time coming.

At the time, pay raises for postal workers were almost unheard of: After 21 years on the job, noted The New York Times, a letter carrier would only earn $2,266 more than their starting salary. Though unionized, postal workers were forbidden from negotiating for pay raises due to cost of living. There was no chance to earn overtime, and many workers had to find a second job to make ends meet.

To top it off, life as a letter carrier was unforgiving. The work was physically demanding, and even experienced employees had no idea how many hours they would work. They waited in break rooms for long periods, hoping to be called for a few hours of delivering the mail. By 1970, their turnover rate was 23 percent.

Tensions boiled over when Congress proposed raising their own salaries by 41 percent in early 1970—but only offered postal employees a 5.4 percent raise. Furious letter carriers in New York called a meeting of the local branch of the National Association of Letter Carriers union on March 17, 1970 to demand a strike.

But the union refused to strike. Union leaders agreed there were legitimate problems, but pointed out that it was illegal for federal workers to strike. (It still is.) Members took a vote. It was close—1,555 for a strike, 1,055 against. However, a group of pro-strike workers led by Vincent Sombrotto defied their union and decided to stop work the next morning.

This “wildcat” strike—one that goes against the wishes of the union leadership—meant that Sombrotto and his colleagues lacked official support for their actions. But they had many supporters elsewhere: other discontented letter carriers from coast to coast.

As letter carriers took to the streets of Manhattan and stopped delivering mail, others joined in. Thirty other cities’ workers walked out, too, and soon at least 150,000 letter carriers—over 200,000, by other counts, often members of other unions—walked off the job.

It was the largest ever walkout of federal employees, and its effects immediately rippled through the nation. At the time, notes the National Postal Museum, letter carriers handled 270 million pieces of mail a day. With no one to deliver them, documents critical to government, finance and other industries sat unprocessed in Postal Department handling facilities.

Letter carriers were just the tip of the iceberg. According to historian Philip F. Rubio, sympathetic bosses allowed some picketing postal workers to clock in and out before heading out to the picket lines and the strike “became a rank and file…revolt.”

“No one had any idea of the chaos that would soon stifle the post offices, the post boxes, the airports, the railroad stations, the stores,” wrote The Guardian’s Alastair Cooke. Weeping women waiting for mail from Vietnam and poor people who needed their welfare checks descended on local post offices, he reported, and businesses announced they might go out of business if the strike continued.

The strike affected another area of life, too: the draft. At the time, the Vietnam War was still raging and draft notices were sent through the mail. Young men now had no idea if they would be called up to war or exempted from it.

“I’m not a rabble rouser or anything like that,” Martin Conroy, a postal clerk from New Jersey who struck in solidarity with the New York workers, told the Philadelphia Inquirer. “Frankly, I’m not the picketing type. But they keep putting us off, and this is the only way we can get any reaction.”

With at least 30 percent of the nation’s letter carriers on strike, the entire postal system began to sink to its knees. Finally, the reaction came—from President Nixon himself. He declared a national emergency and called in U.S. military reservists despite worries that federal action might prompt an even larger strike.

Soon, the National Guard was delivering the mail. Ironically, many of the National Guardsmen were postal carriers who had signed up as a second job. Others struggled to adjust to the difficult sorting tasks and hard delivery work and developed sympathy for the striking workers.

After eight days, convinced by assurances that a deal had been struck with the federal government for a more significant raise, the strike ended. In reality, there was no deal. But when postal employees went back to the job, Nixon’s government gave the workers an immediate pay hike that was also retroactive. A year later, when the U.S. Postal Service was formed, postal unions were given the right to negotiate their salaries and working conditions.

One thing Nixon never did was cancel the national emergency. And his use of an executive order to prompt a military response was not exactly popular. It sparked a special investigation by Congress, and presidential use of both executive orders and national emergency declarations is still hotly debated as a possible overreach of executive power today.

Though Nixon drove a hard bargain while the strike was in progress, his government did not retaliate against those who walked out. Not a single striking worker was fired. Nobody was fined or jailed for acting against federal law. But when labor laws were amended in 1978, allowing collective bargaining for federal workers, provisions that made it illegal to strike stayed in the law.

It’s still against the law for federal workers to walk out on the job. But the strike reminded the government—and the nation—of the power of rank-and-file workers. “Finally,” declared the AFL-CIO, the largest federation of unions in the United States, “the Post Office Department figured out it needed postal workers.”