Under Boris Johnson, the UK has been one of Kyiv's staunchest allies, providing weapons and aid while publicly condemning Putin's invasion.
The crisis is also fuelling the cost-of-living crisis that has risen straight to the top of the UK's political agenda.
As foreign secretary, Truss has already crossed swords with Russia. Here's what she has said about her Russian counterparts, and the public criticism she has faced in return.
What Truss has said about Russia
Truss has been outspoken since Russia started massing troops on Ukraine's border at the end of 2021.
Amid a flurry of diplomatic activity as Western allies from Nato and the EU tried to warn off Russia from invading, Truss said in January: "Russia is the aggressor here. They have amassed a huge number of troops along the Ukrainian border and in illegally annexed Crimea.
"There is no justification whatsoever for Russia’s bellicose stance towards Ukraine. It is unprovoked and it is part of a wider pattern of behaviour by the Kremlin, reliant on disinformation and mistrust to seek to gain an upper hand."
At a tense meeting between her and her Russian counterpart Sergei Lavrov in February, she said they "didn’t like what I had to say".
However, her confrontational approach was somewhat marred by a few verbal trip-ups.
During the meeting, Russian media claimed Lavrov asked Truss if she recognised Moscow's claim of sovereignty over Rostov and Voronezh, two regions in the south of the country where the Kremlin had been building up its forces and both of which have been part of Russia for centuries.
Russian media reported that Truss responded Britain would never recognise them as Russian, and had to be promptly corrected by her ambassador.
Watch: Who is Vladimir Putin?
Britain dismissed the comments as propaganda and said Truss had simply misheard
Truss was also mocked by Russia after she mistakenly referred to the Baltic Sea as the Black Sea when she said: "We are supplying and offering extra support to our Baltic allies across the Black Sea."
After the war started she vowed a tough response and joined with the UK's allies in imposing sanctions on Russia.
In April she said the "age of engagement with Russia is over."
She has described the acts of the Russian government as criminal and said "not a word" president Vladimir Putin says can be trusted.
She said the deliberate targeting of civilians and infrastructure "amounts to war crimes" and "we will ensure Putin’s regime is held accountable."
In her leadership campaign, she has emphasised her strong rhetoric to Tory voters.
Writing in the Telegraph she said she would "go further as prime minister" by declassifying more intelligence to expose Russian misinformation while also repeating a promise to increase defence spending to 3% of GDP by 2030.
"As prime minister, I will be Ukraine’s greatest friend – following in the footsteps of Boris Johnson – and commit completely to ensuring Putin fails in Ukraine and suffers a strategic defeat, and that Russia is constrained in the future."
Her language has won her friends in Ukraine and the foreign secretary Dmytro Kuleba backed her to replace Johnson praising her "mettle, inner steel, and clarity of purpose."
What Russia has said about Truss
Russia has reacted angrily to Truss's statements and has gone as far as suggesting her belligerence has heightened the chance of nuclear war.
At the end of February, Putin ordered Russia’s nuclear deterrent on high alert after "absolutely unacceptable" statements by Truss, but the Kremlin didn't reveal what she had said to trigger this response.
After the frosty meeting between Truss and Lavrov, he described her as "between the dumb and the deaf".
He claimed Truss did not listen to Russia’s position and the UK was unprepared for the talks.
After the meeting Russian media mocked her as "Lady No" and said she only came to Moscow to "show off, to show how macho she is".
The Russian foreign ministry went even further with spokeswoman Maria Zakharova describing her as "bloodthirsty and extremely destructive".
In May, Russia claimed Truss was "very belligerent" and wanted to "prolong" the conflict in Ukraine.
The Russian ambassador to the UK, speaking to the BBC, said: “She’s very belligerent, your minister of foreign affairs, Liz Truss.
"She’s neither a professional military man or she’s not (been) for a long time at this position.
"But she’s very belligerent. If she would like to continue the war she will prolongate the conflict."