Why do European nations need US involvement in Ukraine's security efforts?

European nations need US involvement because the US remains the most powerful country with the strongest military in the world. As the British Prime Minister Keir Starmer noted, "a U.S. security guarantee is the only way to effectively deter Russia from attacking Ukraine again." While European leaders have committed billions in aid to Ukraine and considered contributing peacekeeping forces, they view US support as essential to shore up their own security. This was evident when European leaders held an emergency meeting in Paris, concerned about being excluded from peace negotiations between Russia and the US.