Answer:
To encourage Americans to believe they were making real progress
Step-by-step explanation:
From the sentence given, "...the crushing counteroffensive on the part of the great armies of Russia against the powerful German army.", the most likely intended effect of "crushing counteroffensive" and "powerful" is meant to encourage Americans to believe they were making real progress.
This is because, the term used shows that there is progress being made against a powerful foe.