Belgium and the Franco-Prussian War
Belgium was not a belligerent in the Franco-Prussian War (1870–1871), as the country was neutral throughout the war, but was heavily influenced by the social and political effects of the conflict. An invasion of Belgian territory by either side was widely feared in 1870 and the Belgian Army was mobilised.
Source: Wikipedia — Belgium and the Franco-Prussian War (CC BY-SA 4.0)