How did Stevenson and Donovan try to delay Hitler's invasion of Russia?

The British had warned Stalin about Hitler's imminent invasion, but Stalin didn't believe the Brits, and they couldn't reveal the source of their information. Churchill had a plan to delay Hitler's scheduled invasion, and Donovan went to Eastern Europe to put it in place. He met with King Boris of Bulgaria and got a short delay in German troops being able to pass freely through. In Belgrade, Prince Regent Paul was told that the Germans would only pass through on their way to invade Russia, and the prince was an anti-Bolshevik. Donovan let it be know that the Untied States would only support those who opposed Hitler.