Trench warfare
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.

Geography: The confined, static, and subterranean nature of trench warfare resulted in it developing its own peculiar form of geography. In the forward zone, the conventional transport infrastructure of roads and rail were replaced by the network of trenches and trench railways. The critical advantage that could be gained by holding the high ground meant that minor hills and ridges gained enormous significance. Many slight hills and valleys were so subtle as to have been nameless until the front line encroached upon them. Some hills were named for their height in metres, such as Hill 60. A farmhouse, windmill, quarry, or copse of trees would become the focus of a determined struggle simply because it was the largest identifiable feature. However, it would not take the artillery long to obliterate it, so that thereafter it became just a name on a map.
The battlefield of Flanders presented numerous problems for the practice of trench warfare, especially for the Allied forces, mainly British and Canadians, who were often compelled to occupy the low ground. Heavy shelling quickly destroyed the network of ditches and water channels which had previously drained this low-lying area of Belgium. In most places, the water table was only a metre or so below the surface, meaning that any trench dug in the ground would quickly flood. Consequently, many "trenches" in Flanders were actually above ground and constructed from massive breastworks of sandbags filled with clay. Initially, both the parapet and parados of the trench were built in this way, but a later technique was to dispense with the parados for much of the trench line, thus exposing the rear of the trench to fire from the reserve line in case the front was breached.

In the Alps, trench warfare even stretched onto vertical slopes and deep into the mountains, to heights of 3,900 m (12,800 ft) above sea level. The Ortler had an artillery position on its summit near the front line. The trench-line management and trench profiles had to be adapted to the rough terrain, hard rock, and harsh weather conditions. Many trench systems were constructed within glaciers such as the Adamello-Presanella group or the famous city below the ice on the Marmolada in the Dolomites.

Observation: Observing the enemy in trench warfare was difficult, prompting the invention of technology such as the camouflage tree.

No man's land: The space between the opposing trenches was referred to as "no man's land" and varied in width depending on the battlefield. On the Western Front it was typically between 90 and 275 metres (100 and 300 yd), though only 25 metres (30 yd) on Vimy Ridge.
After the German withdrawal to the Hindenburg Line in March 1917, no man's land stretched to over a kilometre in places. At the "Quinn's Post" in the cramped confines of the Anzac battlefield at Gallipoli, the opposing trenches were only 15 metres (16 yd) apart and the soldiers in the trenches constantly threw hand grenades at each other. On the Eastern Front and in the Middle East, the areas to be covered were so vast, and the distances from the factories supplying shells, bullets, concrete and barbed wire so great, trench warfare in the West European style often did not occur.

Weaponry: 

Infantry weapons and machine guns: At the start of the First World War, the standard infantry soldier's primary weapons were the rifle and bayonet; other weapons got less attention. Especially for the British, what hand grenades were issued tended to be few in numbers and less effective. This emphasis began to shift as soon as trench warfare began; militaries rushed improved grenades into mass production, including rifle grenades.
The hand grenade came to be one of the primary infantry weapons of trench warfare. Both sides were quick to raise specialist grenadier groups. The grenade enabled a soldier to engage the enemy without exposing himself to fire, and it did not require precise accuracy to kill or maim. Another benefit was that if a soldier could get close enough to the trenches, enemies hiding in trenches could be attacked. The Germans and Turks were well equipped with grenades from the start of the war, but the British, who had ceased using grenadiers in the 1870s and did not anticipate a siege war, entered the conflict with virtually none, so soldiers had to improvise bombs with whatever was available (see Jam Tin Grenade). By late 1915, the British Mills bomb had entered wide circulation, and by the end of the war 75 million had been used.
Since the troops were often not adequately equipped for trench warfare, improvised weapons were common in the first encounters, such as short wooden clubs and metal maces, spears, hatchets, hammers, entrenching tools, as well as trench knives and brass knuckles. According to the semi-biographical war novel All Quiet on the Western Front, many soldiers preferred to use a sharpened spade as an improvised melee weapon instead of the bayonet, as the bayonet tended to get "stuck" in stabbed opponents, rendering it useless in heated battle. The shorter length also made them easier to use in the confined quarters of the trenches. These tools could then be used to dig in after they had taken a trench. Modern military digging tools are as a rule designed to also function as a melee weapon. As the war progressed, better equipment was issued, and improvised arms were discarded.

A specialised group of fighters called trench sweepers (Nettoyeurs de Tranchées or Zigouilleurs) evolved to fight within the trenches. They cleared surviving enemy personnel from recently overrun trenches and made clandestine raids into enemy trenches to gather intelligence. Volunteers for this dangerous work were often exempted from participation in frontal assaults over open ground and from routine work like filling sandbags, draining trenches, and repairing barbed wire in no-man's land. When allowed to choose their own weapons, many selected grenades, knives and pistols. FN M1900 pistols were highly regarded for this work, but never available in adequate quantities. Colt Model 1903 Pocket Hammerless, Savage Model 1907, Star Bonifacio Echeverria and Ruby pistols were widely used.
Various mechanical devices were invented for throwing hand grenades into enemy trenches. The Germans used the Wurfmaschine, a spring-powered device for throwing a hand grenade about 200 m (220 yd). The French responded with the Sauterelle and the British with the Leach Trench Catapult and West Spring Gun which had varying degrees of success and accuracy. By 1916, catapult weapons were largely replaced by rifle grenades and mortars.
The Germans employed Flammenwerfer (flamethrowers) during the war for the first time against the French on 25 June 1915, then against the British 30 July in Hooge. The technology was in its infancy, and use was not very common until the end of 1917 when portability and reliability were improved. It was used in more than 300 documented battles. By 1918, it became a weapon of choice for Stoßtruppen (stormtroopers) with a team of six Pioniere (combat engineers) per squad.