Trench Warfare
Breaking through the stalemate
Military tactics before World War I had failed to keep pace with advances in technology. These changes resulted in the building of impressive defence systems, which out-of-date tactics could not break through for most of the war. Barbed wire was a significant hindrance to massed infantry advances. Artillery, vastly more lethal than in the 1870s, coupled with machine guns, made crossing open ground very difficult.
The Germans introduced poison gas; it soon became used by both sides, though it never proved decisive in winning a battle. Its effects were brutal, causing slow and painful death, and poison gas became one of the most-feared and best-remembered horrors of the war. Commanders on both sides failed to develop tactics for breaching entrenched positions without heavy casualties. In time, however, technology began to produce new offensive weapons, such as the tank. Britain and France were its primary users; the Germans employed captured Allied tanks and small numbers of their own design.
http://en.wikipedia.org/wiki/World_War_I