Immerhof protected blockhouses that were constructed during a war that lasted how long?
Ouvrage Immerhof:  In addition, Immerhof protected other Maginot works in the vicinity: the casemates of Kanfen, the infantry shelter of Stressling, the observation point and shelter of Hettange-Grande, and a number of nearby blockhouses constructed during the Phoney War. Phoney War: The Phoney War (French: "Drôle de guerre" ; German: "Sitzkrieg" ) was an eight-month period at the start of World War II, during which there were no major military land operations on the Western Front.
eight-month period