Actually these are two questions in one. I want to express that once something was done, then something else became possible. The first question is, how to say that if that actually happened, and the second when it never happened but could have happened.
Is this correct?
Once the Germans crossed the river Dnepr, they were able to encircle the Soviet forces.
What about this?
Once the Germans would cross the river, they would have been able to encircle the Soviet forces.