At CNN in Washington, Jim Sciutto’s dimly lit office is both man cave and shrine to a foreign correspondent who has reported from more than 50 countries. A typewriter he bought on Portobello Road during a decade in London. Photos he took in Afghanistan and Ukraine. A Vietnamese newspaper account of the time he rode over the South China Sea on a US spy plane. A corked bottle of water from his trip to the north pole in a US nuclear submarine. A fragment of the Black Hawk helicopter destroyed in the raid that killed Osama Bin Laden. “I’m not sure I should have that,” Sciutto confesses, “but I do.”

There is also an important fragment of newspaper, a gift from Sciutto’s grandfather containing a quotation from the author and journalist Eric Sevareid: “What counts most in the long haul of adult life is not brilliance or charisma or derring-do, but rather the quality the Romans called ‘gravitas’: patience, stamina and weight of judgment. The prime virtue is courage, because it makes all other virtues possible.”

When Sciutto’s father died, the quotation was on the funeral mass card.

It was during an assignment that the idea for a book came to Sciutto, 54, CNN anchor and chief national security analyst. As Russian tanks rolled across the Ukrainian border and missiles fell on Kyiv, he sensed a clean break from the post-cold war world he grew up in. He remarked, live on air, that this was a “1939 moment”. What did he mean by those ominous words?

“You have a territorially aggressive leader in [Russian president Vladimir] Putin who’s willing to use force to change borders, and has done,” says Sciutto, who this week publishes The Return of Great Powers: Russia, China, and the Next World War. “You have some in the west who recognise that and others who say just give him this much and it’ll be fine.

“It does have echoes of 1939 and you’ve already experimented with this where you give a little bit of Georgia, you give a little bit of Ukraine and then he calculates, ‘I can take more’ and even when he fails, he tries again. He’s trying again in Transnistria now, to take a little slice of Moldova.”

Then there is China, which also feels wronged by history and the west.

“If you look at Putin’s maps of Europe, the echo is [Chinese president] Xi [Jinping’s] maps of Asia, the nine-dash line. It’s just made up historical justifications for territorial aggression. Then on our side of it you have folks who say: ‘That’s not our war, it’s too far away, just give them a little bit, actually we can work with him.’

“There’s a bit of [1930s British prime minister] Neville Chamberlain in that. Listen, should we all understand why folks don’t want to get in a bloody war with Russia? Absolutely. Or send their sons and daughters to die? 100%. Chamberlain saw world war one and said: ‘I don’t want to have another,’ and you get that. The trouble is, can you actually have peace in your time given the track record, or are you just waiting for the next war, the next land grab?”

A strand of leftwing thought opposes the military-industrial complex (a phrase coined by a Republican president, Dwight Eisenhower) and points to misadventures in Vietnam, Afghanistan and Iraq. But now an allergy to foreign entanglements is all the rage on the populist far right, led by Donald Trump, who espouses “America first” isolationism. Keeping the US out of a third world war has become a staple of his campaign speeches as he seeks to regain the White House.

For Sciutto, there are echoes of Charles Lindbergh, the celebrated aviator who became the leading spokesperson of the America First Committee in the buildup to the second world war.

“You could quote directly from Lindbergh’s speeches and it seems almost verbatim to what you hear today,” Sciutto says.

“I get the argument that no one wants a war and, God knows, you don’t want a nuclear war with either [China or Russia]. On the flip side, there’s no intellectual consistency to that rightwing Republican view because what you’ll often hear them say is: ‘Well, Ukraine’s not our war but maybe China is.’”

Many Republicans are turning their backs on Ukraine but vowing to defend Taiwan from a Chinese invasion. Sciutto continues: “The trouble is, China is watching the response to Ukraine. What’s the fundamental difference? If it’s about sovereignty, you have a sovereignty argument in both places.

“If it’s about trying to stand up to taking territory by force, you have it in both places. Explain to me the difference there. Or even if you relate it to Israel, there’s no intellectual consistency. What’s the argument to defend one but not the other? I suppose looking for intellectual consistency in Washington is a big ask.”

Even worse, he argues, is a false narrative about the Ukraine war.

“I’ve had these conversations on the air with some GOP lawmakers. They’ll be like: ‘Listen, there’s a lot of killing on both sides here.’ I was like: ‘Do you remember how did the war start? Who rolled across the border?’ ‘Well, if we arm them, lots of people are dying here.’ ‘You do remember how this war started and what the fundamental conflict is over?’ A lot of the arguments just don’t stand up to the facts.”

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Republican intransigence on Capitol Hill is having consequences on the battlefield as Russia makes territorial gains and Ukraine runs low on ammunition. The White House describes the situation as “dire”. Last month, Politico reported: “Four American senators recounted a story Ukrainian officials told them at the Munich security conference: a soldier in a muddy trench with Russian artillery exploding nearby, scrolling on his phone for signs the US House would approve military aid.”

Sciutto has been in contact with soldiers, families and others in Ukraine. “They were absolutely terrified that they’re going to be abandoned,” he says. Based on conversations with Ukrainian and European sources, he believes there is a “real danger” Ukraine will lose the war if the US cuts off funding.

Such an outcome would be followed intently in Beijing and Taipei. In his book, Sciutto writes that a war over Taiwan would look very different. Instead of tank battles, artillery barrages and trench warfare, planners foresee lightning air and sea combat, with rapid waves of missiles, anti-satellite weapons and cyber-attacks. Biden has in effect abandoned strategic ambiguity by vowing to defend Taiwan with US troops.

Sciutto notes: “Biden has created, in effect, a new red line there by saying the US will defend them militarily. Not everyone believes that and I go to Taiwan and I ask people that and they’re not so sure. But it’s an open question.”

What would Trump do? In Sciutto’s book, John Bolton, formerly national security adviser, recalls a stunt Trump would perform in the Oval Office: “He would hold up the tip of his Sharpie pen and say: ‘That’s Taiwan. See this Resolute Desk, that’s China.’” His point was that Taiwan is too small to successfully defend itself and too small for the US to care.

As Biden often notes, such debates are putting US credibility on the line. Sciutto had spells in Britain and Hong Kong and was chief of staff at the US embassy in Beijing from 2011 to 2013. During the presidency of George W Bush, he spent a lot of time in Europe. What did he observe about how the world views the US?

“There’s an endless US-bashing that goes on. You’re either too weak or too strong or too involved or not involved enough, some of which just comes with being the richest country in the world and the most powerful military that talks a big game about solving all the world’s problems. To some degree, I’ve been hearing this for years, but I will say that the last decade or two hasn’t improved the US record or soft power abroad. I can say that pretty safely.”

American polarisation doesn’t help. It used to be said that politics stops at the water’s edge. The world knew more or less what to expect from Democrats and Republicans. But just as Biden and Trump radically differ on abortion, crime, guns, healthcare and immigration, so their foreign policy agendas give any foreign diplomat whiplash.

Sciutto observes: “This is the danger. Because foreign policy has become another partisan issue in the country, each election can bring a 180-degree turn in how the US behaves in the world. That has to make our partners say: ‘Right, so Joe Biden is going to abide by article five [Nato’s collective defence clause] but Trump isn’t, where does that leave me? That means we’ve got to make a plan ourselves.’ You hear that more. [The French president Emmanuel] Macron has said it out loud. Others have said it out loud: ‘We’ve got to make our own way.’”

In the week that Trump wrapped up the Republican presidential nomination, forcing his former UN ambassador Nikki Haley to end her campaign, the stakes have been clarified. In his book, Sciutto notes that Trump has expressed admiration for Putin, Xi and even Adolf Hitler. Presidential elections are seldom decided by foreign policy considerations, but the consequences of who owns the nuclear codes could be incalculably far-reaching.

“This election has a real and very defined choice for voters as to what role they want the US to play in the world and how they want to react to this great power competition,” Sciutto says, “because whatever you think of the politics, what Trump has said and done in actions during his first presidency and his positions going into this race show that he somehow envisions a friendly relationship with Xi and Putin.”

In his book, Sciutto quotes a senior US official, who served under Trump and Biden, as saying that in a second Trump term “the US will be out of Nato”. Bolton agrees: “Nato would be in real jeopardy. I think he would try to get out.”

Sciutto adds: “That’s the explicit position of the presumptive Republican nominee, which is the opposite of Biden’s approach to this. That’s a real choice. It’s not going to be a kind of subtle rebalancing. It’s going to be, we’re going to go this way or that way in terms of how we deal with Russia, China and these other places. It’s a real choice.”

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Sciutto is old enough to remember the fall of the Berlin Wall and the political scientist Francis Fukuyama’s prognostications on the “end of history”. Sciutto’s book leaves no one in any doubt it did not turn out that way. It reports on US concerns in 2022 about the possibility Russia was preparing to use a tactical nuclear weapon in Ukraine. James Stavridis, a former Nato supreme allied commander, tells him America is already engaged in a “hybrid proxy war” with Russia.

Sciutto takes no pleasure in playing Cassandra, warning of a world that for all its 21st-century sophistication and irony is backsliding towards Greek tragedy.

“There’s a sadness about it for me personally, because I’m far from a warmonger,” he reflects. “The reason I spent the whole last chapter in the book talking to folks around the world about how to avoid open conflict is because I certainly don’t relish it.

“I don’t want my kids to fight in a war. I don’t want to live in a place where it’s not safe to go to parts of Europe or Asia, for that matter. I brought my family to Beijing for two years and they went to school there and they speak Chinese. I don’t want it to be a world that is divided along those lines, but the sad fact is that’s where we’re heading unless we find a way to navigate away from it.”

• The Return of Great Powers is published in the US by Dutton