What was Trench warfare fought due to new?

Answer 1

To answer the question "why" requires an understanding of where trench war came from, what it meant, and ultimately why it faded away. Here's an explanation that's probably more than you expected but should answer your question.

Trench warfare adapted the line-and-column philosophies that evolved out of Napoleonic warfare to the firepower realities of the Industrial Age. Beginning in the mid 1800s, rifled small arms greatly increased accuracy of fire. Rapid-fire rifled artillery, self-contained cartridges, and ultimately the machine gun all contributed to an escalation of firepower.

Under such conditions, the notion of presenting an exposed line of battle would be suicidal, so tactics called for rapid advance through enemy territory. This approach worked well enough when facing forces that were at significant disadvantage in terms of weaponry, such as was the case with many campaigns during the colonial period in Africa and Asia.

When opposing forces closely matched in terms of weaponry met in battle, however, the model broke down. Large armies fielded in the late nineteenth and early twentieth centuries were simply too large to quickly overrun, and any delay bought the defender time to bring firepower to bear. When that happened, soldiers advancing en masse could be easily annihilated.

Origins

To protect their troops (and it is worth noting that the intention of protecting troops was to keep them viable for future attacks, not primarily or even particularly to save their lives), commanders increasingly came to rely on entrenchment. Soldiers standing in trenches could fire their weapons while standing yet expose only their heads and arms to enemy fire. When fronted by barbed wire, or bamboo palisades like those used by the Maori in New Zealand, trenches were extremely hard to take.

Techniques

"Trench warfare" includes the defensive aspects of this technique, which afforded all of the advantages cited above. However, trench warfare is also characterized by the plight of the attacker facing such a situation. As recently as 1916, the only feasible way to take an enemy trench was to storm it by force of arms.

Direct Assault

Storming trenches was extremely dangerous in the best of conditions, and went something like this:

1. An attack would generally begin with massed artillery fire directed against the target area in the hopes of breaking up defenses. In practice, this alerted the defenders to precisely where the attack would come, and since the artillery had to stop before the attackers arrived, there would be ample time to move in reinforcements.
2. At a chosen time, typically denoted by the blowing of whistles, soldiers in the attackers' trenches would leap "over the top" and charge as fast as they could across the field. Their forward progress would invariably be slowed by debris, bodies from previous assaults, barbed wire, and other obstacles.
3. The defenders, having recovered from the artillery assault and been reinforced, would bring their rifles and machine guns to bear on the advancing forces--which, it must be noted, numbered in the thousands and sometimes in the hundreds of thousands and could not be missed--and open fire. Defending artillery would also begin firing.
4. Enormous numbers of attacking troops would die. During the First World War, it was not uncommon for attacks lasting less than a few hours to results in hundreds of thousands of losses, primarily among the attacking forces.
5. Occasionally, attackers would get close enough to the trenches to throw grenades, clearing holes in the defending troop lines, and leap into the trenches, engaging the remaining defenders with rifles and bayonets. At this point, the warfare within the trench returned to the same roughly even level as pre-trench engagements.

   Far more often, officers would signal a retreat, and the attackers would flee back to their trenches, leaving behind thousands of dead and wounded comrades.

The odds of successfully attacking a trench could be affected to a degree by a few factors, such as the accuracy of pre-attack bombardment and the quality of the troops holding the trench.

However, the odds were so overwhelmingly against the attacker that virtually any attack was destined to fail until the defending military had lost virtually all ability to make war, i.e. to resupply its forces with new soldiers, food, and ammunition. This relationship between defenders and attackers is seen throughout the American Civil War, the Russo-Japanese War, and the First World War; in each case, the attackers eventually overwhelmed the defenders by force of arms but suffered horrible losses to do so.

The Stoßtruppen Model

In the last years of World War One, with millions of soldiers killed attempting to storm Allied trenches, the German High Command envisioned a new offensive technique to break the stalemate of trench warfare. German stoßtruppen, or shock troops, were specially trained infantry who operated in small teams.

The stoßtruppen model used this approach:

1. A short, directed artillery barrage consisting of mixed heavy explosive and poison gas shells would weaken the exact section of the trench to be attacked.
2. Shock troops would advance under cover in small teams to pre-arranged locations, avoiding engagement and concentrating near strongpoints such as machine gun nests, etc.
3. Using a Bangalore torpedo--essentially a very heavy explosive fired through a section of metal tubing--each stoßtruppen unit would hit its assigned strongpoint.
4. Massed German infantry would make a general attack along the line in the wake of the initial strike, spared much of the firepower previously directed against them because of the stoßtruppen attacks.

Properly deployed, stoßtruppen showed enormous success in destroying trenches. Unfortunately for the Germans, by the time that these techniques were perfected, the ranks of the German Army were already greatly reduced, and taking the best troops away for shock troop training only further weakened the lines.

The entry of the United States into the war brought more than a million new soldiers into the Allied ranks, tipping the scales heavily in their favor and giving them the numbers needed to storm the trenches in the traditional manner.

Decline

In the early 1900s, neither armored vehicles nor true warplanes were available. Early tanks were slow, vulnerable to artillery, and broke down frequently. Aircraft of the era could be engaged effectively by riflemen on the ground.

Between the First and Second World Wars, however, everything changed. Armored vehicles in general, and tanks in particular, became increasingly fast, heavy, and reliable. At the same time, advances in aviation saw the advent of heavy bombers that could drop ordinance with a great deal of accuracy from altitudes that put them out of the range of most ground forces.

By the time of the German blitzkreig, trenches, which had offered so much protection against direct-fire weapons, were literally targets and easily overrun. Moreover, fast-moving tanks could simply bypass them, rendering static formations like the French Maginot Line obsolete. Entrenchment continued to have strategic implications following the Second World War, but its value continued to wane with the development of new and better aircraft.

In the waning years of the twentieth century, the First Gulf War demonstrated a final defeat for the evolution of trench warfare. Iraqi military units that had invaded Kuwait dug their armored and infantry forces in behind heavy sand fortifications to await an anticipated American-led armored advance. By most reports, their fortifications were impressive in the model of industrialized trench warfare, and while the Iraqis were aware of certain limitations in their equipment, they nonetheless anticipated that the combination of concealment and protection against direct fire would offer substantial advantages.

U.S. warplanes, however, enjoyed complete air superiority. For more than six weeks before initating a ground assault, the allies bombarded Iraqi positions almost nonstop. When U.S. and allied armored forces did finally advance against the demoralized and battered Iraqis, thermal imaging equipment provided locations of Iraqi tanks behind the dunes and specially designed armor-piercing rounds called sabot penetrated the sand as if it did not exist, destroying fortified vehicles before they could return fire.

With trenches no longer providing concealment nor defense for modern armies, the value of trench warfare was eliminated once and for all.

Answer 2

Trench warfare was fought due to new technologies and weapons.