Trench warfare
Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. On the Western Front in 1914–1918, both sides constructed elaborate trench, underground, and dugout systems opposing each other along a front, protected from assault by barbed wire, mines, camouflaged trapping pits, and other obstacles. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties.
Trench warfare
Only starting in 1916, on the battlefield of Verdun, came the use of portable small radio stations that could transmit short Morse codes. Radio frequency overlap occurred with neighboring divisions until precise regulation and radio discipline developed. Also, enemy interception was common until effective codebook practices were enforced. These problems were not completely resolved until the war's end.