While Ukraine’s counteroffensive against Russia’s invasion has global attention on the battlefield, President Volodymyr Zelenskyy’s government is also busy advancing a diplomatic initiative: a peace summit to build momentum and cohesion among international partners on its 10-point peace plan. The United States should be a leader in backing this diplomatic effort — which is on the agenda this weekend in multilateral talks in Saudi Arabia. Broadening international buy-in for Ukraine’s peace plan serves U.S. interests. It can short-circuit less constructive peace initiatives and reinforce a cardinal international norm: An aggressor that launches an unprovoked war can't expect to set the terms for peace afterward.