The strategic importance of the Belfort fortification system became apparent in the Franco - Prussian War of 1870 .
The strategic importance of the Belfort fortification system increased as a result of the loss of Alsace and Lorraine as a result of the war .
The strategic importance of the Belfort fortification system became apparent in the Franco - Prussian War of 1870 , and increased as a result of the loss of Alsace and Lorraine as a result of the war .