One afternoon in late spring I met Tony Blair in his central London office to talk about when it is right for a nation to go to war. It was 20 years almost to the day since he had made a speech in Chicago outlining the rules by which western nations could establish when—and how—they would intervene militarily in other countries in order to prevent ethnic cleansing, mass atrocities and genocide.

The context was Kosovo, and Blair’s argument was that “acts of genocide can never be a purely internal matter,” and we now lived “in a world where isolationism has ceased to have a reason to exist.” He conceded that if the west “sought to right every wrong” it “would not be able to cope,” and so set out five tests for so-called “liberal intervention.”

These criteria would play at least a notional role in every decision by the UK to go to war—and to not go to war—over the next two decades. The five tests encapsulated a dominant view across the west in those heady post Cold-War days that a handful of powerful democracies should stand ready to prosecute what Blair described as “just war, based not on any territorial ambitions but values,” of human rights and democracy.

Few speeches by a British prime minister have been as influential, so the tests are worth recapping: “First, are we sure of our case? […] Second, have we exhausted all diplomatic options? […] Third, on the basis of a practical assessment of the situation, are there military operations we can sensibly and prudently undertake? Fourth, are we prepared for the long term? […] And finally, do we have national interests involved?”

Calling a question a test, however, doesn’t make the answer to it any less subjective—or contentious. While there was relatively little opposition to Britain’s intervention in Kosovo in 1999 and in Sierra Leone a year later, the same could not be said for Iraq. Blair still won’t admit that Iraq missed his tests, although many others certainly do. But in talking, he did admit for the first time that “there were elements that were missing” from his original doctrine. He now wanted “very strongly” to add two more tests, narrowing the circumstances in which going to war is wise.

The first—and most controversial—is to consider “if you’re going into a country, where there are going to be strong, Islamist influences at play… whose very purpose is to destabilise what you’re trying to do… who are prepared to kill and die in pursuit of that.” This, he said, is “what we found in Iraq and Afghanistan, what the Russians and Iranians and others found in Syria, and what the [Saudi-led] coalition forces are finding in Yemen.” His use of the very specific word “Islamists,” when “insurgents” would make the same point more generally, is telling.

Blair’s second new test is public opinion at home. “I think it is difficult to do this if it’s going to be a long-term project, and your own country is divided about it.” Especially, he added, if there is a change in government “and the people who come after you have no particular interest in seeing that long-term project through.” These extra tests might seem almost custom-made to acknowledge the disaster of Iraq. All he wants to say about that conflict however is that it was “not a bad or ignoble mission.”

A long-retired politician belatedly finding theoretical holes in his military doctrine would normally be of interest only to historians. But this retired politician casts a uniquely long shadow over British foreign policy because every UK political leader since has defined their own position relative to the Blair doctrine.

David Cameron wanted to be “realistic not utopian.” “If we have learnt anything…” he said in 2007, “it’s that you cannot drop a fully formed democracy out of an aeroplane at 40,000 feet.” Theresa May went further in Philadelphia in 2017, rebuking Blair by arguing that the “days of Britain and America intervening in sovereign countries in an attempt to remake the world in our own image are over.” On the Labour side, Ed Miliband positioned himself in subtle opposition to Blair, before passing the reins to Jeremy Corbyn who deems him a “war criminal.”

And yet, the Blair doctrine has never quite been banished. Four years after Cameron dismissed it, he was launching his own avowedly humanitarian mission in Libya. May “doesn’t do the parachuting-into-Benghazi thing,” one of her officials told me—a dig at Cameron’s presumptively triumphal trip—but by last year she too had changed her mind, telling the UN General Assembly we “must have the will and confidence to act when the fundamental rules that we live by are broken,” and adding a Blair-like flourish: “when barbarous acts and aggression go unchecked—dictators and terrorists are emboldened.”

Only last autumn, the then-defence secretary, Gavin Williamson, sat down with his closest advisers to discuss a speech outlining a new defence doctrine. “I rather admire Blair’s Chicago speech,” he said, according to someone who was in the room. “I know it’s not flavour of the month anymore, but how could we recast it?”

Talking to senior diplomats and officials at No 10, the foreign office and the ministry of defence, it is clear that the idea of Britain intervening militarily for humanitarian reasons is still deeply held. But they harbour two doubts, the first of which chimes with Blair’s new tests: can they persuade us, the public? Their second doubt, though, is different. With a rising China, a resurgent Russia and a volatile and isolationist America, has the world changed too much? If liberalism is “obsolete,” as Vladimir Putin recently put it, then is liberal interventionism over as well?

***

In around 50AD the Greek philosopher Onasander wrote a short treatise called The General, drawing on centuries of Aristotelian thinking. “The causes of war,” he wrote, “should be marshalled with the greatest care; it should be evident to all that one fights on the side of justice.” A general commencing hostilities, “should call heaven to witness that he is entering upon war without offence… and is not deliberately seeking… to ruin the enemy.”

There is a thread that runs from such ancient “just war” arguments all the way through to the Blair doctrine. But over the 2,000 years since, the pendulum has swung back and forth many times between a readiness to intervene for moral ends and a presumption that the proper way to treat foreign lands is to leave them alone.

That second view became deeply entrenched when a line was drawn under the Thirty Years’ War with the Peace of Westphalia in 1648. After a conflict in which pious justifications had licensed all manner of atrocities, avoiding war must have seemed like the over-riding humanitarian objective—and guaranteeing every state’s sovereignty over their own affairs the smartest way to secure it.

By the 19th century, however, ideas about a more proactively moral foreign policy were, for better or worse, flickering into life. In the Opium Wars, London’s effective role was as a sort of imperialist drug-pusher, and yet Lord Derby insisted the case for taking on China was not a mere matter of national interest, but the high liberal principle of free trade.

Gladstone’s Midlothian campaign denounced not only the “Bulgarian Horrors” of the Ottomans but also Disraeli’s indulgence of them. Later, amid the gold mines and the concentration camps of the Boer War, there was lofty talk—or, if you prefer, imperialist cant—about securing equal rights for “Uitlanders.”

In the 20th century, the pendulum swung firmly back towards national interests. The League of Nations committed members to “respect and preserve… the territorial integrity and existing political independence of all members.” Respect for sovereignty and “spheres of influence” arguably helped prevent the Cold War turning hot, but also often meant human rights got trampled on.

Occasionally, the superpowers would get more assertive—President Kennedy vowed to “bear any burden, meet any hardship… oppose any foe to assure the survival and the success of liberty.” But before long, the “military advisers” he sent to Southeast Asia in the name of freedom were beset by mission creep, and the US was engulfed in a Vietnam War which set the pendulum swinging again.

There are two important lessons to remember here: one, there is always a back and forth. As Blair’s long-time No 10 chief of staff Jonathan Powell put it to me, “you do it, it’s sensible, then you overreach. And then you go into a cycle where you don’t do it at all.” The other, as the historian Christopher Coker told me, is that powerful nations need a “permissive environment” in order to act. In the 19th century, there were no great powers able to stand in Britain’s way, nor did it need to spend money on defending itself at home. It wasn’t until the end of the Cold War that the west would again summon the means or the will to at least consider a more consistently proactive doctrine—not that there has ever been much consistency in practice.

***

The first time I went to Mogadishu, in 2007, I met a young man called Khalid Abdullahi. He lived with more than 20 members of his family in what my notes from the time describe only as a “concrete, dirt-floored shack” on a “winding dusty track.” There were cactus trees outside, chickens pecking in the dirt. Like most buildings in the Somali capital, parts of it weren’t standing anymore. There was a hole in the roof where something had come crashing through 14 years before. Somalia had been involved in one conflict or another since Khalid was a boy. He led me to a back room, where a mattress was propped up at an angle against the wall, with an object I couldn’t make out in the darkness protruding behind it. He pulled the mattress away from the wall, revealing a large, black slab of fibreglass. It had dents and scratches, gaps where it looked as if a small window might once have been, and it curved towards the front. Khalid shyly turned to me for approval. I was staring at the nose cone of a Black Hawk helicopter, one of two shot down in Mogadishu in 1993. This was what had come through the roof. “The idea of Britain intervening militarily for humanitarian reasons is still deeply held” The US had sent 25,000 troops to Somalia in December 1992 as part of a 37,000-strong UN force—a military response to a humanitarian catastrophe. The Somali government had collapsed, famine had swept the country and warlords were, supposedly, stealing much of the aid that had been sent. Both the UN, led by the first African secretary-general Boutros Boutros-Ghali, and the US, led by the first President Bush, saw Somalia as an opportunity to demonstrate a new post-Cold War world order.

It was a disaster. The US attempted to kill one of the warlords accused of siphoning off aid. He fought back, the Black Hawk crashed into Khalid’s house, and 18 US Army Rangers and around a thousand Somalis were killed during the fighting which followed. Bill Clinton, who had dodged the Vietnam draft in his youth and had inherited this intervention along with the White House from Bush Sr, pulled his troops out on 3rd March 1994.

The Americans’ crass mixing of militarism with aid had doomed their mission, and also exposed one of the perennial problems of liberal intervention. However universalist the initial intentions, domestic pressures force western political leaders to give special weight to their own forces. When they come into harm’s way, the western powers either over-react in a way which discounts local lives, or else—as in this case—they cut and run, which can sometimes set different problems in train.

A month after the last American troops left Mogadishu, the genocide began in Rwanda. Over the course of three months, at least 800,000 people were killed. Here, the UN and the US—both so keen to intervene in Somalia—ignored the warnings from the head of the UN peacekeeping mission in Rwanda, as well as their own intelligence. Two years later, similar reticence in Bosnia was followed by the Srebrenica massacre.

That 1990s journey—from Somalia, through Rwanda, to Bosnia—formed the background to Kosovo, and Blair’s attempt to forge a new doctrine for the 2000s. In 1998, Serbia’s President Slobodan Milosevic launched a crackdown, driving Kosovar Albanians from their homes in a programme of ethnic cleansing. When peace talks failed, Nato launched airstrikes. And a few weeks into the campaign, Blair flew to Chicago to make a speech.

***

The odd thing about the Blair doctrine is that, according to him, he didn’t mean to create it. “People still sort of think about it as a defining speech,” he told me. “I didn’t think of it in quite that way at the time.” The Nato airstrikes weren’t working, partly because the US was unwilling to threaten to commit ground troops. That’s why the speech took place in Chicago—its aim was not theoretical, but urgently practical: to put pressure on the White House. In that sense, it succeeded. “It ended up as the ‘splash’ in the New York Times: ‘Blair pushes Clinton to be a real man,’” recalled Jonathan Powell. “Clinton called up in a fury from Air Force One and really tore into Tony.” But it worked. Clinton promised ground troops and the threat was enough to force Milosevic to concede.

A single success, then, from which a doctrine was forged. “The theory came out of the practice rather than the other way around,” said Powell. “Perhaps this was the mistake. But then we believed in its general applicability.”

Others sensed this widening out from the Kosovo case—and worried. Especially in the foreign office, which had been cut out of the speech’s preparation. Blair’s foreign secretary, Robin Cook, strongly supported the Kosovo intervention, but had profound fears about the emerging broader doctrine. His private secretary, Sherard Cowper-Coles, recalls him saying: “this is adventurism, it’s mad.”

Historian Lawrence Freedman, who had helped Blair and Powell frame the tests, saw them as having the opposite purpose. They were “designed to show the limits of intervention,” he told me, whereas for Blair, “Chicago was his statement about why it’s important to intervene rather than why you should be cautious.” (Freedman’s concerns about the warping of principled war aims would be reaffirmed years later when he served on John Chilcot’s Iraq Inquiry. It identified a gulf between the “moral case” Blair was making in public, and his private and unswerving commitment to the US summed up in a 2002 letter to George W Bush: “I will be with you, whatever.”)

The man himself, though, still maintains the whole point was to be flexible. “You’ve got to judge each case on its merits,” Blair said. To be “in principle against intervention is as big a mistake as being in principle in favour of it. Which I should say, you know, I’m obviously not. People used to say to me, ‘why aren’t you intervening in Zimbabwe?’ and I said, ‘because there’s no basis for which we could do that successfully.’”

Whatever the doctrine was actually trying to do, however, the fact that the same tests were invoked in the very different contexts of Kosovo, Sierra Leone and Iraq has seen all these interventions lumped together. It is forgotten that some of the Iraq war’s sternest critics—Cook, for example—were also some of the strongest supporters of the interventions in Kosovo and Sierra Leone. After Iraq, intense suspicion set in about anything to do with the Blair doctrine, and the age-old pendulum started swinging towards non-intervention again.

*** If Iraq was a disaster—and it’s worth pointing out that, aside from Blair and Powell, not a single person I spoke to thought otherwise—then the experience of Libya and Syria muddies the waters. Libya was initially held up as an example of swift western action to prevent an atrocity: Muammar Gaddafi had vowed to attack Benghazi and “show no mercy,” and blocking his path seemed to France, Britain and the US to be a realistic and urgent imperative. But intervention sceptics noted how the rationale changed from the humanitarian emergency to regime change within hours of it beginning. And once the no-fly zone had been introduced and the tide of the war had turned, France, Britain and the US lost interest. Today, years after Gaddafi’s brutal death, the country is still in turmoil, with a barely existent government under attack from an out of control warlord—and civilians and thousands of would-be refugees stuck in the middle. Syria was—is—more complex. It shows the dangers of non-intervention, while also demonstrating that the west is no longer the only military power whose decisions count. Back in 2011, William Hague had told the national security council that intervention to prevent Bashar al-Assad’s brutal crackdown on protests could be “the greatest foreign policy failure since the fall of Singapore.” Two years on, the situation had become more complicated as protests had turned into violent rebellion. The country was a morass, with the Free Syrian Army already losing support and fighters to Islamist rebel groups, some of whom had links to al-Qaeda.

But the regime’s horrific chemical weapons attack on civilians in rebel-held areas breached what President Barack Obama had described as a humanitarian “red line,” and in both the US and Britain there was an urgent debate over how to respond. Plans were drawn up to launch a series of strikes designed to prevent Assad from repeating such an attack. In August 2013, a complacent Cameron recalled parliament assuming that MPs would nod the intervention through, as they had with Libya. But there were no briefings for wavering MPs, and chief whip George Young—told of the recall late, while on holiday in France—had no time to count the numbers, still less change them.

Labour’s Ed Miliband, still defining himself against Blair, proposed a motion that didn’t say “no” but set a range of tests—such as specific findings from UN inspectors—which implied “not yet.” Miliband’s motion passed and Cameron immediately conceded, telling MPs “I get it,” rather than trying again. Hague took defeat “very personally,” said a senior official who worked closely with him, and that affected how the diplomatic staff viewed him. “We’d previously thought he was very sure-footed,” the same official said, but the rushed vote was “a massive misjudgment.”

The ripples around the world, however, affected not just the reputation of one foreign secretary, but that of the west as a whole. Obama had been pressured into holding a similar vote, but support in Congress fell away and the attacks were cancelled. “That moment,” said Freedman, showed that both Britain and America “had lost the urge to try to sort everything out where they could. It demonstrated there were limits on intervention.”

David Hannay, a former British diplomat and one of just four voices raised in favour of the attacks in the Lords, sees it as a profound moment of retreat from the world. “It was the beginning, I’m afraid, of the very sharp decline in Britain’s influence… It showed that we really weren’t up to it.” Much of Whitehall agrees: “It was a turning point,” sighs one senior official in the ministry of defence.

*** Back in the confident dawn of the 1990s, the Blair doctrine was not the only attempt to formalise rules about humanitarian intervention. Roughly in parallel, the UN was devising Responsibility to Protect (R2P), a doctrine that overturns the idea that sovereignty trumps everything. If an atrocity is taking place and a government is unable or unwilling to prevent it—or is responsible for it—it puts an onus on the UN. Crucially, unlike Blair’s doctrine, it is “exclusively about action by international organisations,” says Hannay, who was part of the panel tasked by then secretary-general Kofi Annan with creating the concept. Nor, he adds, is it “normally about military intervention,” but “diplomatic intervention… economic intervention, the usual panoply of things the UN tries to do.” All this will make it sound more palatable than the Blair doctrine to some, but in practice the two concepts have often been muddled. Libya, for example, was held up as an example of liberal interventionism but also, based as it was on support from both the Arab League and the UN Security Council, R2P. It also has its own flaws—for instance, Russia has blocked 13 UN Security Council resolutions on Syria since 2011. There is no failsafe framework to tell the international community when to intervene against atrocities. The best they can do is resist the temptation to lump them all together, and instead answer the difficult judgments involved in each instance with the right mix of experience, caution and humanitarian values.

But there are other constraints as well. In truth, the west could only ever debate intervention as it did in the 1990s and 2000s because it was free to choose what it wanted to do. The new era of Great Power competition changes the calculus. While America and Britain chose not to intervene in Syria—beyond a single night bombing raid ordered by Donald Trump in 2017 after another chemical weapons attack—Russia took a different path. Putin’s decision to commit forces to bolster President Assad showed that the west is no longer the only player.

Meanwhile China, which places a great if selective emphasis on respect for sovereignty for self-serving reasons, has a major -economic, if not military, presence in dozens of countries across Africa. This could not safely be ignored before embarking on any future interventions there.

Intervention is also only possible if you don’t have to worry about your own defences. That, too, has changed recently, with the rising Russian threat, and then the longer-term dangers emanating from China. “Most of us at the top of the MoD,” an official there told me, “believe that, by a long way, the biggest challenge we face is managing state-to-state competition.”

“A Whitehall official sees ‘a collapse of confidence in what the west can do’”

An even more consequential geopolitical change may be the election of an isolationist to the White House. On the first night of bombing in Libya, 114 bombs were dropped. More than 100 of them were American. If the US is reticent about using its hard power, then any military intervention is almost impossible. Besides, would a British government—and a British public—want to fight a war alongside Trump, however moral the cause? Cameron couldn’t make the case for air-strikes against Assad carried out alongside Obama. The test could arrive in the coming months, as the US edges towards a possible dangerous conflict with Iran which would have no humanitarian rationale whatsoever.

What does the future hold for the Blair doctrine, or any other version of liberal intervention? It may not be quite so liberal, for a start. The fear, said one official, is that we’re now in an age of “illiberal interventionism,” with Russia’s actions in Syria the prime example. “You can criticise our interventions,” said Blair, “but ours were at least to remove dictators, and set up a democratic process. [Russia and Iran’s] are to support a dictator and refuse a democratic process.”

In insecure times, it is also naïve to assume that any action the west can muster will be from pure motives either. Humanitarian language may, as it has sometimes in the past, again end up being used to gloss interventions made for more selfish reasons of security. “There are perfectly noble humanitarian reasons for wanting to build stability in the Sahel region,” one senior ministry of defence official told me. “Mali resembles the Balkans of the 1990s.” But the reason why we may intervene is less altruistic: “stopping a new Afghanistan popping up close to Europe.”

“There’s a reason we’ve been debating this for several thousand years,” concludes Powell. “It’s because we can’t actually find the right answer. We do care, we do want to do something about it, but sometimes the consequences are so grave we withdraw from it.” If the west didn’t know that before Iraq, it does now. As one official put it: “You’ve got to use the supremacy well. And we didn’t.”

A pendulum that has always swung, now looks like getting stuck with isolation. “In my darkest moments,” one Whitehall official told me, “I worry that we’re seeing a broader collapse in confidence in what the west can do.” Another wondered aloud what would happen if ethnic cleansing took place in Europe once again. “Would we intervene?” he asked. There was a pause, then a quiet “no.”