Do you have to be right on your forecast to choose the right course of action?

Sometimes you don't need to be right as long as everyone agrees on the same ideas.

I was thinking of Roosevelt, Churchill and Stalin at the beginning of World War II. Did they know how the war was to turn out? Did they know that the Allies would win the war, and did they know that no other alternative to World War II would have worked out better?

• Did Roosevelt know how the Japanese would fight to the death, and that the atom bombs would be developed in time to avoid invasion of Japan and loss of a million American servicemen?
• Did Stalin know that a second front would not be opened until D Day in 1944, and that the USSR would have such heavy casualties and damage to its infrastructure while bearing the brunt of the Nazi war machine?
• Did Churchill know that his party would lose power at the end of the war and that the war would lead to the end of the British empire?

Of course not. They had to have had some opinions of what would happen, but as far as I know about the three of them, they were unlikely to share their private thoughts in the late 1930s with posterity. Still I can assume that they were almost certainly not right about what would happen during the war as they agreed on a united front against the Axis, and a war to the unconditional surrender of Germany and Japan.

They didn't have to be right in their predictions in order to agree on the right course of action.