Which region had previously been claimed by France and Germany and created tension between the two countries prior to World War I

The region that had been claimed by both France and Germany and which created significant tension between the two countries prior to World War I is Alsace-Lorraine. This area is located along the border of France and Germany and carries strategic importance as well as economic value due to its rich industrial and coal-bearing areas.

France controlled Alsace-Lorraine until 1871, when Germany annexed the region after its victory in the Franco-Prussian War. This annexation caused much resentment among the French people and was a major cause of acrimony between the two nations. The desire for the return of Alsace-Lorraine to French control was a significant aspect of French policy and nationalist sentiment leading up to World War I.

When the war began in 1914, reclaiming Alsace-Lorraine was one of France's primary objectives. Following the end of World War I, the Treaty of Versailles in 1919 officially returned the region to France, which was a significant point of national pride for the French and a sore point for the Germans, whose loss of territory contributed to the grievances that eventually fueled World War II tensions.