Germany lost an important World War II ally when

A. France fell to US and British forces.

B. the Soviet Union invaded Germany.

C. the Soviet Union surrendered to US forces.

D. Italy surrendered to US and British forces.

The answer would be D. France was part of the allies with the U. S and Britan. The invasion of the soviet union never caused a loss of an ally and the Soviet Union was a part of the allies.

I believe the answer is: D. Italy surrendered to US and British forces.

The main strategy of the axis power is to let both italy and Japan handle the allies at the western front while the Germans would handle the soviet union on the eastern front. The moment italy surrendered to US and British, Germany had to face armies from both western and eastern front.