 Complete the following phrase:  In December 1916, Füßlein, commander of German mining operations in the salient, had recorded that British deep mining was intended to support an offensive above ground and received three more mining companies, to fight in the British lower as well as the upper mine systems and had gained some success. In April 1917, the 4th Army General Friedrich Bertram Sixt von Armin received information from air reconnaissance that a British offensive was being prepared in the Messines Ridge sector and a spy reported to OHL, that if the offensive at Arras was frustrated, the British would transfer their effort to Flanders. Hermann von Kuhl, the Chief of Staff of Army Group Crown Prince Rupprecht, suggested that the salient around Messines Ridge be abandoned, since it could be attacked from three sides and most of the defences were on forward slopes, vulnerable to concentric, observed artillery-fire. A voluntary retirement would avoid the calamity experienced by the defenders at the 