He also warned against “Ukraine fatigue,” saying it’s “very important to show the UK is in “for the long haul.”
It comes as NATO Secretary-General Jens Stoltenberg said the war “could take years.”
Meanwhile, the UK’s new army chief warned that British troops must prepare “to fight in Europe once again.”
In an article published in London’s Sunday Times newspaper on June 19, Johnson argued that Russian President Vladimir Putin’s recent speech marking the 350th anniversary of Russian Tsar Peter the Great’s birth showed Putin “would not stop at dismembering Ukraine” if Russia prevails in the war.
“Only last week, he compared himself to Peter the Great and arrogated to Russia an eternal right to ‘take back’ any territory ever inhabited by ‘Slavs,’ a doctrine that would permit the conquest of vast expanses of Europe, including NATO allies,” the British prime minister wrote.
Johnson said while Putin’s “total reconquest of Ukraine” had been derailed, “in his isolation, he may still think total conquest is possible.”
He said the UK and its allies need to “steel ourselves for a long war” by making sure Ukraine can strengthen its military capabilities faster than Russia can replace its lost tanks and armour.
Johnson set out a four-point plan, including accelerating weapon supplies to Ukraine and the training of Ukrainian soldiers; constant funding and technical support to help sustain the Ukrainian state; developing the overland routes in and out of Ukraine to counter Russia’s “stranglehold on Ukraine’s economy by blockading its principal export routes across the Black Sea;” and getting food out of Ukraine by supporting the United Nations to negotiate a safe corridor for exports by sea.
But he warned that while “the need to restore food exports could scarcely be more pressing,” none of these steps “will yield immediate results.”
“All will require a determined effort by the UK and our allies, lasting for months and years,” he wrote.
Speaking to reporters earlier on Saturday following his second surprise visit to Ukraine, Johnson said, “When Ukraine fatigue is setting in, it is very important to show that we are with them for the long haul and we are giving them the strategic resilience that they need.”
Gen. Sir Patrick Sanders, who took over from Gen. Sir Mark Carleton Smith as the British Army’s chief of the general Staff, said there’s now “a burning imperative” that the army is prepared for war in Europe.
Writing to troops, Sanders said Russia’s invasion of Ukraine has underlined the core purpose of the British Army of protecting the UK by being ready to fight and win wars on land.
He also said he is the first chief of the general staff since 1941 to “take command of the Army in the shadow of a land war in Europe involving a continental power,” referring to World War II.
“The scale of the enduring threat from Russia shows we’ve entered a new era of insecurity,” he wrote. “It is my singular duty to make our Army as lethal and effective as it can be. The time is now and the opportunity is ours to seize.”
Sanders said there’s “now a burning imperative to forge an army capable of fighting alongside our allies and defeating Russia in battle,” adding, “We are the generation that must prepare the Army to fight in Europe once again.”
NATO chief Stoltenberg on Saturday told German newspaper Bild am Sonntag that the alliance must prepare for the Russia–Ukraine war to “take years.”
A NATO summit in Madrid later this month is expected to agree on an assistance package for Ukraine that will help the country with the move from old Soviet-era weaponry to NATO standard gear, Stoltenberg said earlier this week.
Ukraine vowed on Saturday to prevail against Moscow as it fought Russian assaults near a key eastern city and multiple locations came under shell and missile attacks.
Russian forces were defeated in an attempt to storm Ukraine’s capital Kyiv in March. Russia has since refocused on the Donbas region in the eastern part of Ukraine.
Reuters contributed to this report.