I want to know the answer to the following question:
What factor forced the French to retreat to Metz?
All the background I'm provided with is that "With the defeat of Marshal Bazaine's Army of the Rhine at Gravelotte, the French were forced to retire to Metz, where they were besieged by over 150,000 Prussian troops of the First and Second Armies.". Is that enough to answer the question?
Answer: yes