That helps explain why President Donald Trump’s promise to end the war between Russia and Ukraine (within a day, as he used to say) is proving difficult. His officials say they have achieved “epic” results.

Moscow’s demands are slowing the process and giving it the advantage in talks with a deal-hungry United States, experts said.

The United States said Tuesday that it had reached a tentative agreement for Ukraine and Russia to stop fighting and ensure safe navigation in the Black Sea in separate talks with both sides, but many details were unresolved,