 In the earliest stages of the First World War the German Army 's rapid advance along the North Sea coast found the German Imperial Navy without submarines suitable to operate in the narrow and shallow seas off Flanders . By 18 August 1914 , two weeks after the German invasion of Belgium , the planning of a series of small coastal submarines had already begun . 
 In the earliest stages of the First World War the German Army <a href='null_genetive' title='1'><mark>'s</mark></a>rapid advance along the North Sea coast found the German Imperial Navy without submarines suitable to operate in the narrow and shallow seas off Flanders . By 18 August 1914 , two weeks after the German invasion of Belgium , the planning of a series of small coastal submarines <a href='been_done' title='2'><mark>had</mark></a> already begun . 
In da earliest stages of the First World War the German Army  rapid advance along the North Sea coast found tdaGerman Imperial Navy w/o submarines suitable to operate in thdaarrow and shallow seas off Flanders . By 18 August 1914 , 2 weeks after the German invasion of Belgium , the planning of a series of small coastal submarines done already begun .