Alberto Pezzali / AP

In the fourth volume of Robert Caro’s epic biographical series The Years of Lyndon Johnson, the author writes: “Although the cliché says that power always corrupts, what is seldom said, but what is equally true, is that power reveals.

What is true of power is also true of crises in which political leaders are thrust into the spotlight. How they respond in these moments can reveal elements of their character camouflaged from the public beforehand. What do they say? What should they do? A carefully constructed political image or campaign strategy is suddenly tested by an event that they could not have prepared for.

These are the tests a leader faces in such a situation—and is the test now facing Prime Minister Boris Johnson and the opposition leader, Jeremy Corbyn, following the attack on London Bridge today.

How they respond is a question of judgment. There is no time to stress-test slogans or consult focus groups. Their reaction is instinctive. Do they rise to the occasion or not? In this moment, prime ministers, and those seeking to be prime minister, are presented with the power to lead. How they use it reveals more than dozens of campaign rallies or thousands of leaflets do, because it is them, forced to lead—or unable to.

This afternoon’s assault on London Bridge has become a wearily familiar scene in Britain, as have the words of condemnation and calls for unity from the country’s political leaders that inevitably follow such incidents. There are the expressions of outrage, that the attacker showed cowardice and the emergency responders great bravery. The public is urged to be vigilant, but not let the terrorists win. All of these statements can be—and are—true, and yet they can blur together to the point where they flirt with being meaningless unless a leader is able to capture the moment.

It is worth remembering that this attack comes just two and a half years after the previous frenzy of violence at the same spot in central London, at a similar point in that year’s general-election campaign, when eight people were killed and dozens of others injured using the same methods designed to spread fear: wild, roving stabbings and fake suicide vests for extra terror. The 2017 London Bridge attack came not long after an even bloodier atrocity in Manchester, when 22 people were murdered in a suicide bombing at an Ariana Grande concert. Both attacks happened during that year’s general-election campaign, twice causing the contest to pause, while also throwing new political issues into the mix in ways few expected, from foreign policy to the impacts of austerity. The same may happen again this time, or altogether new issues may emerge. No one knows.

The crucial point, however, is that the atrocities did more than throw up new policy issues. They served to reveal the essence of those vying to lead the country, lifting a curtain on the humans underneath.

What the country saw then—in truth, the campaigns themselves had already revealed elements of these characters—were two entirely different people with different impulses and instincts. Then–Prime Minister Theresa May, a security-conscious former home secretary, buried herself in the work of government, chairing emergency meetings to oversee the response. There were hardly any tweets or other social-media engagement—nothing deemed trivial. It was a time for prime-ministerial behavior, she and her aides believed. While unafraid to attack Labour politically for its response—this was not a politically naive woman—her immediate reaction was to focus on the effort to control the situation.

Corbyn, as leader of the opposition, was free from the constraints of executive power, yet the two crises and his response to them nevertheless revealed core elements of his character as well. His reaction, particularly to the Manchester attack, was to show solidarity. His leadership was emotional, compassionate. The day after that bombing, a vigil was held in Manchester. May stayed in London to oversee the response, sending her home secretary, Amber Rudd, instead; Corbyn made sure he was there.

In some sense, both of these responses are necessary in times of crisis. To some of those around May whom I spoke with after the 2017 campaign, there was a feeling that she, to some extent, had underestimated the need to lead emotionally, and not simply to work harder than anyone else, even if the cause she was working toward was noble and right. At such moments, people want reassurance and comfort; they want to be given a narrative to understand the incomprehensible, to see that someone is in charge. By sending Rudd to the ceremony, May was doing what she felt was right—and this is not to say that that judgment was wrong—but it left a void of symbolic leadership at a time of shock and outrage.

Today’s attack on London Bridge, using the same methods of violence and terror as two years ago, is a challenge to Johnson and Corbyn to show the country who they are, where their instincts lie. Corbyn’s, having been tested in 2017, are well known and are unlikely to change. He is good with people and will not shy away from airing his views on what he sees as the link between terrorism and Britain’s foreign policy, despite the political risk this entails. Indeed, his aides believe that speaking out in this manner is not a risk at all, because his views chime with those of many in the country.

Johnson’s response will be telling: While he has been one of the most well-known figures in British politics for years, he has rarely been tested in a crisis. And the one time he was, during the 2011 London riots, when he was the city’s mayor, he did not overly impress; he was heckled as he made his first appearance after returning slowly—and seemingly reluctantly—from a family holiday.

As a figure whose reputation borders on the clownish, Johnson faces the test of showing something more than optimism and cheeriness, and channeling public anger and upset. How he does so in the coming days—how he uses the power he has won in being prime minister—will reveal.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.