1903, Guggisberg, F.G. , Modern warfare : or, How our soldiers fight 

Ubique [Sir Frederick Gordon Guggisberg] (1903) Modern warfare : or, How our soldiers fight

Brigadier-General Sir Frederick Gordon Guggisberg KCMG DSO (20 July 1869 – 21 April 1930) was a senior Canadian-born British Army officer and British Empire colonial administrator. He published a number of works on military topics and Africa. [Wikipedia]

The book describes an Anglo-German war in the manner of a detailed army manual.

It attracted little attention, although the writer described a German invasion of Belgium and a British warning that any attack on Belgium would be considered a hostile act. In consequence, when the German armies cross the Belgian frontier on July 1st, 1905, ” the telegraph lines from London flash the order to ‘mobilize’ to the Army and Navy in all parts of the United Kingdom. Before midday every corner of the Empire knows that Britain and Germany are at war.” [I.F. Clarke (1965) ‘The Shape of Wars to Come’, History Today, Feb 1, 1965]