A retired spy and his daughter are found slumped on a park bench in Salisbury. Someone tried to kill them. The poison is novichok, a Soviet-era nerve agent. Six months later Theresa May says the would-be assassins are officers with Russian military intelligence. They travelled to the UK as “Alexander Petrov” and “Ruslan Boshirov”.

Not true, says Moscow. Last month the two men appeared on RT, the Kremlin’s external propaganda channel. They denied having anything to do with the bungled attempted murder of Sergei and Yulia Skripal. Yes, they visited Salisbury twice. But they came in March to see the city’s wonderful cathedral and turned back the first time because of heavy ‘slush’ …

The investigative website Bellingcat has revealed the pair’s real identities. “Boshirov” is Anatoliy Chepiga, a GRU colonel. “Petrov”, we learned this week, is Alexander Mishkin, a trained GRU medical doctor. Both are decorated heroes of Russia. As for the slush, Mishkin comes from a remote snow-covered village in the frozen Arkhangelsk region.

Which version to believe? States have always lied about their activities. So, it must be said, have (some) western politicians. But we are now living in an age where malign individuals and authoritarian nations such as Russia are able to spray around lies on a global scale, using the firehose power of Facebook and Twitter.

You don’t have to believe the two Russians. Vladimir Putin’s aim isn’t to persuade the world community that the pair are hapless tourists – though this line works to some degree at home. Putin’s ultimate goal is to confuse. As RT’s boss, Margarita Simonyan, once put it, there is no such thing as truth, only “narrative”.

Facebook Twitter Pinterest Journalist Luke Harding (centre standing) speaking with colleagues in the Guardian office. Photograph: Alicia Canter/The Guardian

As the Guardian’s Moscow correspondent, I watched as the Russian government perfected these techniques for a domestic audience. Over the past decade, the Kremlin has rolled out the same methods abroad. It is part of a wider attempt to reshape the world to Russia’s advantage. Other leaders are using the same authoritarian handbook.

We see rightwing populists trying to create a sovereign version of “reality”. Think of Donald Trump and his claim – easily disproven by photos – that his inauguration crowds were “bigger than Obama’s”. Or Rudy Giuliani, Trump’s loyal aide, who told NBC that the president would not answer questions under oath posed by Robert Mueller, the special prosecutor investigating collusion. “Truth is not truth,” Giuliani said.

Putin, Trump and other unscrupulous people have declared war on critical thinking. For Lenin, truth was subordinate to class struggle. Putin has taken this relativist idea and weaponised it. Russian state TV channels spew out endless conspiracy theories, which are amplified by professional trolls and shared online. The web – a once empowering force – has become a playground for disinformation and malign messaging.

And, as the historian Timothy Snyder argues, post-truth is pre-fascism. Without a basic consensus about facts, science, law and parliamentary politics get corroded. Our democracy becomes degraded. Climate change? A hoax! Collusion? Fake news! Russian hacking of the US presidential election? It could have been anybody!

Trump’s account of the world is false and self-serving. And successful. His Republican supporters live in their own separate knowledge universe, fed and affirmed by Fox News and conservative talk radio. They believe the president is a victim of a deep-state plot. There is divide in thinking – an epistemological barrier that creates hostile tribes, a them and an us.

Facebook Twitter Pinterest ‘Boshirov’ and ‘Petrov’ in Salisbury. Photograph: Metropolitan Police/PA

All of this presents an enormous challenge for journalism. How should reporters respond to this assault on truth? How do we avoid what you might call “versionland” – where rival “versions” are accorded the same status, even if one of them is wrong? How do we hold a civilised conversation?

One possible answer, I think, is good method. As a foreign correspondent, I believed in seeing for myself: travelling to the frontline of a war, or driving to the scene of a natural disaster. We need to talk to all sides. If the facts change, so should our reporting. Fixed ideas are unhelpful. The truth – so best as we can establish it – does exist. Where possible we should uncover it.

The work of journalists is under attack as never before. Our best response is to develop a spirit of solidarity. The Guardian is one of a number of influential media organisations that have begun to work collaboratively. We still want to break exclusive stories. But interrogating complex issues is sometimes best done as part of a global team.

I’ve been lucky to have been involved in many groundbreaking investigations. They include the leak of state department cables; Edward Snowden’s revelations of mass US-UK surveillance, for which the Guardian jointly won a Pulitzer prize; and the Panama and Paradise Papers. Plus stories on Trump’s Russian connections and the links between the Russian ambassador in London and Arron Banks.

We have worked on many of these projects with talented reporters from around the world. Nearly 400 journalists cooperated on the Panama Papers. We swapped information – we called this “radical sharing” – and pooled discoveries and tips. Remarkably we managed to keep our year-long investigation secret: no mean feat given the gossipy proclivities of most hacks, especially after a pint.

When the Panama Papers were published in 2016, they had an enormous impact. David Cameron nearly resigned. The prime ministers of Iceland and Pakistan did quit, with the latter eventually ending up in jail. There were protests in Argentina, Malta, Brazil. At Westminster, the first meaningful steps were taken to make Britain’s offshore former colonies more transparent.

The investigation revealed a bitter truth: that the secret offshore industry is not a minor part of our economic system. Rather, it is the system. The rich and multinational corporations exited from tax a long time ago, leaving the rest of us to pick up the slack. This sense of fundamental economic unfairness explains in part why millions voted for Trump – and Brexit.

At a time when politics seems to be failing, the Guardian is more determined than ever to hold power to account. Paradoxically, this feels to me like a golden age for journalism. The answer to onslaughts from Trump and co is to carry on – to tell true stories, compellingly and well. Most of all we need our readers. Please support our work in dark times, and help light us the way.

Luke Harding is the author of Collusion: How Russia Helped Donald Trump Win the White House, published by Guardian Faber and available through the Guardian bookshop.