Vladimir Putin has paved the way for Ukrainian membership of Nato, Boris Johnson has said.
The war has “vapourised” the case against admitting Ukraine to the alliance, Mr Johnson wrote in an op-ed for the Washington Post.
Ukraine has been seeking to join Nato for years, with calls for admission ramping up since the Russian invasion last year.
Decades of “diplomatic doublespeak” on the subject had led to the war, Mr Johnson said.
The argument that allowing Ukraine to join Nato would be “provocative” to Russia is no longer valid, he wrote, admitting that as Prime Minister he “should never have accepted” this argument.
“We should have insisted on the reality — that the Kremlin had nothing to fear from Nato, because it is a defensive alliance."
He added: “Russian President Vladimir Putin has destroyed countless lives, homes, hopes and dreams.
“He has also destroyed the slightest reason to sympathise with him, or to humour him in his paranoia.”
Ukrainians ended up with “the worst of both worlds”, he argued, with Nato showing enough positivity towards Ukraine for Mr Putin to claim Russia was being threatened in his propaganda, but not enough to actually protect the country or advance its membership.
Other previous reasons given for not admitting Ukraine do not hold up either, the Conservative MP said - such as a lack of support, or insufficient military strength.
“People used to claim that Ukraine wasn’t properly militarily compatible with Nato. Today, Ukrainians are deploying a dizzying variety of equipment from Nato countries, with the utmost skill and bravery,” he wrote.
“There is absolutely nothing that Nato could teach Ukrainians about fighting a war — in fact, there is a lot that they could teach us.”
It comes after the former prime minister made an unannounced visit to Kyiv on Sunday, which he said was at the invitation of Ukrainian president Volodymyr Zelensky.
He also visited the towns of Bucha and Borodyanka, which were occupied by Russian forces last year.
"I welcome Boris Johnson, a true friend of Ukraine, to Kyiv. Boris thanks for your support!" Mr Zelensky wrote on Telegram.
In a BBC documentary aired on Monday night, Mr Johnson claimed Mr Putin threatened him with a missile strike in a phone call before the invasion in February last year, saying “it would only take a minute”.
In response, the Kremlin accused the former prime minister of lying, saying “there were no threats with missiles”.