President Donald Trump said Monday he’s begun arrangements for a face-to-face meeting between Vladimir Putin and Volodymyr Zelenskyy to discuss a pathway to end Russia’s invasion of Ukraine, while affirming that the U.S. would back European security guarantees aimed at preventing Moscow from reinvading its neighbor once the current conflict ends.

Details of the security guarantees and Trump’s efforts to arrange peace talks have continued to evolve in the hours following an extended meeting among Trump, Zelenskyy and other European leaders at the White House.

But as they emerged from their talks, the leaders expressed guarded optimism that Trump could be finding momentum in his quest to fulfill his campaign promise of ending the grinding war.

What to know about efforts to end the Russia-Ukraine war:

Trump spoke with Putin by phone: Trump spoke with the Russian leader during his meeting with European leaders, later saying he’d begun making arrangements for a meeting between Putin and Zelenskyy. Trump called it “a very good, early step for a War that has been going on for almost four years.” It’s unclear whether Putin has fully signed on to such talks.Trepidation from Europe: Many in Europe worry that Trump is pressing Ukraine to make concessions that will only further embolden Putin. NATO Secretary General Mark Rutte said that “if Russia is not playing ball” on direct talks with Ukraine, “the United States plus Europe will do more when it comes to tariffs and sanctions” on Moscow.‘NATO-like’ security guarantees: Trump stopped short of committing U.S. troops to a collective effort to bolster Ukraine’s security. He said instead that there would be a “NATO-like” security presence and that all those details would be hashed out with EU leaders. NATO itself is underpinned by a political commitment by all member countries to come to the aid of any member whose sovereignty or territory might be under attack.

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