Q: Given a premise, an initial context, an original ending, and a new ending, the task is to generate the counterfactual context that is aligned with the new ending. Each instance consists of a five-sentence story. The premise is the first sentence of a story, and the second sentence, which is the initial context, provides more information about the story's context. The original ending is the last three sentences of the story. Also, you are given a new ending that can provide the new story with the same premise. You should write a counterfactual context to explain the small differences between the original and new endings. More specifically, a story context contains the general plot of the story. And a counterfactual context is a slight modification to the initial context. To sum up, you should write the second sentence of a story based on the premise(first sentence) and the new ending(last three sentences) of the story.
Premise: The allies invaded Fortress Europe on D-Day. 
 Initial Context: Since then their progress across France has been swift. 
 Original Ending: They've advanced almost all the way to Germany. One obstacle stands in their way, the Rhine River. A swift victory is assured once past that. 
 New ending: They've advanced almost all the way to Paris. One obstacle stands in their way, the Germany army. A swift victory is assured once past that. 
A:
Since their progress across France has slowed down.