structural:expansion, structural:summarization
Wire

The use of lines of barbed wire, razor wire, and other wire obstacles, in belts 15 m (49 ft) deep or more, is effective in stalling infantry travelling across the battlefield. Although the barbs or razors might cause minor injuries, the purpose was to entangle the limbs of enemy soldiers, forcing them to stop and methodically pull or work the wire off, likely taking several seconds, or even longer. This is deadly when the wire is emplaced at points of maximum exposure to concentrated enemy firepower, in plain sight of enemy fire bays and machine guns. The combination of wire and firepower was the cause of most failed attacks in trench warfare and their very high casualties. Liddell Hart identified barbed wire and the machine gun as the elements that had to be broken to regain a mobile battlefield.

A basic wire line could be created by draping several strands of barbed wire between wooden posts driven into the ground. Loose lines of wire can be more effective in entangling than tight ones, and it was common to use the coils of barbed wire as delivered only partially stretched out, called concertina wire. Placing and repairing wire in no man's land relied on stealth, usually done at night by special wiring parties, who could also be tasked with secretly sabotaging enemy wires. The screw picket, invented by the Germans and later adopted by the Allies during the war, was quieter than driving stakes. Wire often stretched the entire length of a battlefield's trench line, in multiple lines, sometimes covering a depth 30 metres (100 ft) or more.

Methods to defeat it were rudimentary. Prolonged artillery bombardment could damage them, but not reliably. The first soldier meeting the wire could jump onto the top of it, hopefully depressing it enough for those that followed to get over him; this still took at least one soldier out of action for each line of wire. In World War I, British and Commonwealth forces relied on wire cutters, which proved unable to cope with the heavier gauge German wire. The Bangalore torpedo was adopted by many armies, and continued in use past the end of World War II.

The barbed wire used differed between nations; the German wire was heavier gauge, and British wire cutters, designed for the thinner native product, were unable to cut it.