Why did the Soviet Union mistrust the US and Britain during World War ll?

Stalin didn't just mistrust the western allies, he mistrusted everybody. Stalin had to fight for the leadership of the USSR after Lenin and had many in the party who opposed him. He dealt with these by arrest, deportation and murder, just as he did with the officer corps of the armed forces (a factor that led to the slow Soviet response to invasion by the Germans) and the Kulak landowners created by Lenin (that led to mass starvation). He had also mad a 'pact' with the Germans over Poland and this Non-aggression pact had ultimately led to nothing but deception on Hitler's side. Stalin knew Communism was an 'enemy' of capitalism and his capitalist allies indeed mistrusted him, especially Churchill. Stalin felt that the lack of a credible second front was in order to weaken the Soviets for a continued invasion by the British and USA. This again ultimately led to the annexing of the Warsaw Pact countries and formation of the 'iron curtain' in Churchill's Words.