Wiktionary
n. 1 A military policy of terrorising an enemy, especially through its civilian population. 2 A general atmosphere or situation of awfulness or dread.
Wikipedia
Schrecklichkeit (German "terror" or "frightfulness") is a word used by English speakers to describe an assumed military policy of the German Army towards civilians in World War I. It was the basis of German actions during their march through Belgium in 1914. Similar policies were followed later in France, the Russian-held area of Poland, and in Russia.
Under the traditional "laws of war" in Europe, military activity was to be confined to the regular, uniformed armies of each side. Civilians were to take no part in fighting, neither attacking or being attacked. Thus, when the uniformed army troops in a nation or region were all defeated, resistance was supposed to end. In practice, informal resistance by "partisans", "guerrillas", or "francs-tireurs" often continued after the "regular" troops had given up or withdrawn. These fighters, never officially enrolled in an army, and not wearing uniforms to identify them as combatants, attacked occupying troops by ambushes, surprises, and sniping. Conducting such operations out of uniform was considered perfidy, and those involved could be executed.
Occupying armies sometimes responded to such attacks by reprisals against the local population: execution of local inhabitants, whether known to be guerrillas or not, or destruction (usually by burning) of homes and other structures.
German army doctrine in force at the time called for such reprisals to be performed immediately and severely in any case of civilian resistance. It was argued that such schrecklichkeit would end resistance quickly with relatively little bloodshed, whereas restraint would encourage resistance, leading to greater destruction and death.
When Germany invaded Belgium in 1914, the German high command expected to sweep through the country with negligible opposition. The German army was many times larger and stronger than the Belgian army, and the Germans therefore thought that any resistance by Belgium would be futile. German leaders had even suggested to the Belgian government that in the event of war, the Belgians should just line up along the roads and watch the Germans march through. Belgium's refusal to accept these German presumptions and its resistance to the German advance came as a surprise, and disrupted the German timetable for advancing into France.
This frustration was communicated to the German troops in Belgium. Anything which delayed the German advance was to be crushed mercilessly. The Belgians were viewed as irrational and even treacherous for their opposition.
This led to exaggerated suspicions among German commanders of Belgian civilian resistance. It is possible that some Belgian civilians engaged in resistance, though none is documented. It is certain that on several occasions, German commanders declared (probably in unconscious error) that such acts had occurred when they had not.
The Germans responded to these perceived acts of resistance with harsh measures. In several villages and towns, hundreds of civilians were executed. Many buildings were put to the torch. Priests thought guilty of encouraging the resistance were killed. Violence by German soldiers against Belgians, such as rape, was ignored or not seriously punished. The Belgian city of Leuven was largely destroyed. One German officer later wrote about the town, "We shall wipe it out...Not one stone will stand upon another. We will teach them to respect Germany. For generations people will come here and see what we have done".
These actions, taken in a period of near-panic as the German forces desperately tried to carry out their flanking march before Allied forces could respond, proved to be a propaganda disaster for Germany. The reports of them caused a wave of indignation which aided the Allied cause.