In 1994 the writer Julian Evans went on a 10-day cruise down the Dnipro River. Ukraine had won its independence three years before. The journey took Evans along an ancient route used by the “restless Vikings” who established Kyiv. His ship – the Viktor Glushkov – stopped off at Crimea and Yalta. Its final destination was the glittering Black Sea port of Odesa.

Evans was a veteran traveller. Nonetheless, the city was “unlike any place I had visited”, he writes – a “country beyond the back of a wardrobe” where anything could happen. It had merchants’ houses, acacia trees, a dandyish 19th-century opera and ballet theatre, and wide neoclassical boulevards. It was ostentatious and self-made. There was kolorit: exoticism and flash.

After seven decades of communist decay, mafia goons roamed the streets. In its gangster heyday, Evans recalls, Odesa was a “dusty Sleeping Beauty being kissed awake by a guy in a blacked-out Mercedes with a handgun in the glove box”. Regional crooks dominated. The city was full of stories, and known for sarcasm and wit. It was home to Isaac Babel, the poet Anna Akhmatova, and Bob Dylan’s Jewish grandparents.

For Evans, it was the beginning of a three-decade love affair with Odesa and its people. He returned five years later to make a radio programme about Alexander Pushkin, another resident, and got chatting to a waitress called Natasha. He invited her for a drink. A courtship followed – with walks, Moldovan wine and letters from London. Evans converted to Orthodoxy and married Natasha in a monastery.

Later they spent languid summers in Odesa with their two children. Meanwhile, Ukraine was changing from a Moscow-style bandit state to a fledgling democracy and what Evans calls an “emerging unlikely paragon of freedom”. Ukrainians were willing to fight for their rights, in contrast to their supine neighbours. There were street revolts in 2004 and 2014: against election cheating and a corrupt Kremlin-backed government.

Evans’s new book is a brilliant portrait of a country and a city now under Russian attack. Most nights Iranian drones and Iskander ballistic missiles wallop Odesa, bringing murder and mayhem. The blossom-scented harbour promenade and Potemkin steps that enchanted Evans echo to Ukrainian anti-aircraft fire. Having failed to seize Odesa in the early weeks of his invasion, Vladimir Putin is busy destroying it.

The author’s relationship with the city lasted longer than his marriage, which eventually fizzled out. In 2015 he toured the then eastern frontline near Mariupol, following Russia’s covert part-takeover of the industrial Donbas region. In November 2022, he arrived in a missile-struck Odesa during a blackout. His mission this time was humanitarian: to donate a Nissan pickup to the outgunned Ukrainian army, plus tourniquets.

Evans is a wonderful writer and observer, a stylist as elegant and gloriously free-wheeling as the late Jonathan Raban. Each paragraph has a lapidary charm. There is plenty of history too, effortlessly told. Before the Vikings, Odesa attracted Greek colonists. Other settlers include Italian merchants and Tatars. It was always a place of migrants, and more than the southern imperial riviera capital of Catherine the Great.

The memoir offers a shrewd analysis of Ukraine’s present struggle and what it means. The British media has frequently got it wrong, Evans suggests. In 2014 it depicted Moscow’s attempted takeover as a fight between “Ukrainian nationalists” and “pro-Russian separatists”. The actual fault line was economic: between the country’s poor and super-rich. Ukraine’s oligarchs lit the “garbage fire” that made Russia’s war possible, Evans argues.

The nation’s fate now hangs in the balance. On the battlefield Russia is advancing. Internationally, there is a strong whiff of betrayal in the air, as Trump returns to the White House and Ukraine’s allies get tired. Trump has promised to resolve the war – borne from centuries of Russian oppression – in “24 hours”. It seems likely he will bully Volodymyr Zelenskyy into a “peace deal” on Moscow’s brutal terms.

Evans characterises the invasion as fascist. The Kremlin’s goals of expansion and assimilation by force are identical to those of the Nazis in the 1930s, “if less efficiently executed”, he writes. It is “plain sordid imperial theft” and “robbery with violence”. If Putin prevails in Ukraine, he will gobble up other European nations. China and North Korea – which has sent troops to bolster Russia’s army – may launch grabs of their own.

After three years of all-out war, Ukrainians are exhausted. The west talks about moral principles, one disillusioned soldier tells Evans, but sends too few weapons to make a difference. “Ukraine’s spirit is flagging from loneliness,” he says. If Europe and America believe in freedom and democracy “we should not shy away from putting our soldiers on the ground”. There is little prospect of that happening.

During a visit in May, Evans finds young women sitting in ones or twos in a beachside restaurant in Odesa, enjoying the evening breeze. The Black Sea is cleaner than before. Dolphins appear. Missing from this dreamy scene are men. Some are at home, hiding from conscription. Others are fighting in trenches on the eastern front. Thousands lie in cemeteries under yellow and blue flags.

Luke Harding’s Invasion: Russia’s Bloody War and Ukraine’s Fight for Survival, shortlisted for the Orwell prize, is published in paperback by Guardian Faber

• Undefeatable: Odesa in Love & War by Julian Evans is published by Scotland Street Press (£24.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply