President Trump's shutdown ended, temporarily at least, on Friday. But right before it did, we got a fleeting glimpse of the kind of power America's workers could wield if they got truly organized.

There are multiple threads to what went down last week. On Thursday, two competing bills to reopen the government failed to pass the Senate. One bill was designed to cater to Trump's demands, the other to Democrats' approach. Both fell short, but the Democrats' bill got more votes — meaning it benefited from GOP defections. Trump was already getting creamed in the polls over the shutdown. And the idea of his own party starting to jump ship apparently pushed the White House to a new level of panic on Friday. But something else happened that day, too.

Air traffic controllers, ground down by weeks of work without pay, finally walked off the job in sufficient numbers to cause major delays at some of the country's busiest airports — including La Guardia in New York City, Hartsfield Jackson in Atlanta, Newark's Liberty International, and Philadelphia International.

It was a critical mass of workers calling in sick, as opposed to any coordinated activity, that did it. The National Air Traffic Controllers Association was careful to specify that it did not "condone or endorse" any coordinated activity "that negatively effects the capacity of the National Airspace System." But the union didn't stop there: "We have warned about what could happen as a result of the prolonged shutdown. Many controllers have reached the breaking point of exhaustion, stress, and worry caused by this shutdown."

President of the Association of Flight Attendants-CWA Sara Nelson, who had already aired the possibility of a flight attendants strike to protest the shutdown, made things even more explicit Friday morning. "We're mobilizing immediately," she told New York. "Showing up to work for what? If air traffic controllers can't do their jobs, we can't do ours."

You can debate which of these two developments weighed heavier on Trump's team in the moment. But at a minimum, the threat of air travel stopping along America's Eastern Seaboard threw "gasoline on the fire," as several Capitol Hill sources put it to The Daily Beast.

And you can see why.

Suppose the shutdown hadn't ended. Suppose air traffic controllers did walk off the job in a coordinated effort, across the country. Imagine flight attendants did the same. And air travel throughout the country just ... stopped. Imagine the kind of political firestorm that would spark.

"Is this what we want?” Kevin Drum asked at Mother Jones. "Do we really want the folks who run our air travel system to have this kind of power? I'm not so sure that would be a great thing."

One of the great paradoxes of American society is that people we don't usually think of as having leverage actually do — if they mobilize as a group to use it. Air traffic controllers and flight attendants are two of the most obvious groups. But this also includes police and firefighters, restaurant staff, taxi and Uber drivers, teachers, garbage workers, shipping workers, manufacturing workers, and more. They are all overwhelmingly middle-to-lower class, and can be found across public and private employment. Whether they're paid well or treated decently by their employers are questions that rarely trouble elites, policymakers, or U.S. politics in general. The continuance of their labor is taken for granted precisely because it's critical for the daily rhythms of American society and commerce. That they might all say "screw it" and walk off the job en masse is considered unthinkable, precisely because it would stop the entire American economy.

You can hear that existential dread in the National Air Traffic Controllers Association assurance that of course they would never endorse or condone an organized refusal to work. You can hear it as well in all the hand-wringing over what teacher strikes will do to students. You'd think the right to not work if you're not getting paid or treated well — much less not getting paid at all — would be foundational in a country that considers itself capitalist. But the unspoken assumption is that the labor these Americans do is too important to allow them that freedom.

This coercion rarely becomes clear in the U.S. economy. It's largely carried out through property rights: work or you and your family will starve, or go without medicine or health care. A lot can be squeezed out of a workforce this way. But Friday's brief air travel collapse was a reminder that even desperate people can be pushed too far.

This is why the labor movement battles of the early 1900s were so massive, violent, and bloody. America scarcely has any cultural memory of that time anymore. But that was when Americans pushed to the brink by unfair working conditions and wages first realized how much they could accomplish if they went on strike.

On Friday, working Americans were briefly reminded of the power of walking away.