(Bloomberg) -- At home, UK Prime Minister Keir Starmer has framed his trip to meet Donald Trump as a bid to save Ukraine and the trans-Atlantic alliance. At the White House, he’ll argue that the US president needs Europe to come out a winner.

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Starmer is in Washington on Thursday to hold his first face-to-face meeting with Trump since the populist Republican’s election to a second stint in the White House. The stakes couldn’t be higher for the prime minister, who faces a career-defining crisis, with generational implications for the North Atlantic Treaty Organization and the British-American “special relationship” at its core.

While Starmer can gesture toward the bust of Winston Churchill recently restored to the Oval Office, he’s hoping the UK’s fresh plans to boost defense spending to 3% of economic output over the next decade will prove more convincing to Trump. His main objective is to secure an American “backstop” for any truce between Russia and Ukraine, a cause French President Emmanuel Macron also pushed in his own White House visit earlier this week.

European leaders are scrambling to slow down talks over Ukraine’s fate after Trump’s stunning decision this month to open two-way peace negotiations with Russian President Vladimir Putin. Ukrainian leader Volodymyr Zelenskiy is expected to meet Trump on Friday, in the next procession of anxious American allies trying to hold ties together three years after Moscow’s full-scale invasion of its neighbor.

Britain and France have proposed deploying “reassurance” forces to Ukraine in the event of a cease-fire with Russia. But the pair of founding NATO members want US security guarantees, including air power and intelligence, to deter future aggression from Moscow.

Without them, European officials argue that Trump will have a harder time securing the legacy of “peacemaker and unifier” that he aspired to in his inaugural address last month. Ukraine could reject the deal and fight on. Putin might feel emboldened and strike again, risking a wider European war between nuclear-armed states.

“The security guarantee has to be sufficient to deter Putin from coming again,” Starmer told reporters on the plane to Washington. Still, remarks by Trump moments later at the White House underscored the challenge facing the Europeans.