British Prime Minister Keir Starmer arrived at the White House on Thursday with one overarching message for U.S. President Donald Trump: U.S. military backing is vital to ensure a lasting peace in Ukraine.

“The security guarantee has to be sufficient to deter [Russian President Vladimir] Putin from coming again,” Starmer said ahead of his arrival in Washington. “Because my concern is if there is a cease-fire without a backstop, it will simply give [Putin] the opportunity to wait and to come again because his ambition in relation to Ukraine is pretty obvious for all to see.”

Starmer has suggested a willingness to provide British troops as part of a peacekeeping force in Ukraine but has said that London’s involvement would be contingent on the United States providing air, logistics, and communications support. To demonstrate his seriousness, Starmer on Tuesday announced the biggest increase in British defense spending since the end of the Cold War, a rise from 2.3 percent of the country’s GDP to 2.5 percent by 2027.

Trump has previously urged NATO allies in Europe to take on a greater share of the defense burden. But it appears that Starmer’s actions were still not enough to fully sway the U.S. president.

During his first formal U.S. cabinet meeting on Wednesday, Trump dismissed the idea of U.S. security guarantees, saying, “We’re going to have Europe do that.” He doubled down on that position on Thursday, adding that a cease-fire in Ukraine is needed before they can decide on a peacekeeping force. When asked on Thursday whether the United States would support British peacekeeping troops in Ukraine if they were attacked, though, Trump said, “If they need help, I’ll always be with the British.”

Read more in today’s World Brief: Starmer Urges Trump to Commit to U.S. Security Guarantees for Ukraine.