Final answer:
The occupation of the Rhineland by the German military was problematic because it violated the Treaty of Versailles, threatened the security of European nations, particularly France, and marked a significant step towards World War II.
Step-by-step explanation:
The German military occupation of the Rhineland was a significant problem for European nations primarily because the occupation violated the Treaty of Versailles. The Treaty of Versailles, which concluded World War I, imposed many demands on Germany, including the demilitarization of the Rhineland to create a buffer zone between Germany and France. However, in March 1936, Hitler flouted these restrictions and sent German troops into the Rhineland, thus breaking the terms of the peace treaty.
This action not only breached the agreement but stoked fears and tensions in neighboring countries, especially France, as it threatened their security and the peace of Europe. The remilitarization of the Rhineland was a pivotal moment in the lead-up to World War II, as it highlighted the failure of the international community to enforce the Treaty of Versailles and emboldened Hitler to pursue further aggressive policies.