Final answer:
Luxembourg is the country where the Moselle River forms its border with Germany, and the Sauer flows eastward across it, also along the border with Germany.
Step-by-step explanation:
The country where the Moselle River forms its border with Germany, and where the Sauer River flows eastward across the country, along the border with Germany, is Luxembourg. Luxembourg is a small landlocked country in Western Europe, bordered by Belgium, France, and Germany.
The Moselle River forms a natural border between Luxembourg and Germany for a stretch, and the Sauer is a tributary of the Moselle, also serving as part of the border with Germany. Luxembourg's involvement in European history is significant, as it has been influenced by and has contributed to the historical events surrounding Germany and its neighbors.