Final answer:
Germany declared war on France two days after declaring war on Russia because of the invasion of Belgium.
Step-by-step explanation:
The reason Germany declared war on France two days after declaring war on Russia was because of the invasion of Belgium. Germany's plan for war on France involved moving westward and then sweeping south to head for Paris. However, this required the German armies to march across Belgium, which refused to allow them entry due to a treaty that guaranteed their independence. As a result, Germany decided to invade Belgium, which led to France being attacked as well.