King Albert I, who succeeded King Leopold II in 1909, led a widely respected constitutional monarch. Albert introduced many modern reforms, not only in Belgium but also in its African colony.

In 1839, Belgium’s neutrality was guaranteed by the European powers. As tension in Europe was rising in the 1910s, Belgium declared neutrality again in November 1913 in apprehension of a World War. On August 2, 1914, Germany asked from the Belgium government free passage through Belgium toward France. Germany wanted to first attack France in the first month of the war, according to its Schlieffen Plan, named after the German general who strategised a plan to first overtake France in the first month of the war, thus leaving Britain and Russia unprepared, and unwilling, to attack Germany. 

The Belgium government rejected this plan, which led to the invasion of German troops in Belgium and Luxembourg. For two months, almost all of Belgium was occupied in spite of the opposition of the Belgian Army. Some parts of the extreme west remained free, where Belgium troops were concentrated, but for the next four years the frontline basically remained the same.

The Germans in World War I were stopped by the Allied Powers in the two battles of the river Yser, in Belgium, with much loss of life to the Belgians. The Belgium population was deprived of its rights under German occupation. King Albert I stayed in Belgium with the troops to lead his army while the government stayed in Le Havre, in France. In the spring of 1918, the German troops made a last attempt to win the war but were stopped, not only by the Allies but also by the newly arrived U.S. troops, putting an end to the war.