Final answer:
Germany was forced to fight a two-front war during World War I, facing France in the west and Russia in the east. The Germans had hoped to swiftly defeat France and then turn their attention to Russia, but the invasion of Belgium brought condemnation and prolonged trench warfare on the Western Front.
Step-by-step explanation:
The nation that was forced to fight a two-front war during World War I was Germany. Anticipating a conflict with both France and Russia due to the French-Russian alliance, German military strategy involved fighting a defensive war against Russia in the east while launching an offensive against France in the west. They hoped to quickly defeat the French and then direct their full military might against Russia. However, this plan was complicated by the need to invade Belgium to attack France, which painted Germany as the aggressor and led to Belgium's occupation by German forces.
On the Eastern Front, despite the Russian army being poorly equipped and trained, they initially caused significant challenges for the Germans. On the Western Front, the German advance was halted at the Battle of the Marne, leading to a prolonged and grueling stalemate marked by trench warfare. The situation was further complicated by the myriad of alliances that had been formed, which meant that local conflicts had the potential to escalate into a larger war involving many nations.
The central powers, notably Germany, became diplomatically isolated, with enemies in the west (France) and east (Russia), which significantly determined the course and challenges Germany would face throughout the war.