Trench warfare is where two armies are at war, and they both are fighting from trenches, or long ditches in the ground. It is a very old type of warfare, but it was at it climax in WWI. After that the ability to fight on the move known as "Mobility" made trench warfare out dated. It is still taught to some extent for defending a small position. But for the most part it has been abandoned. Check wikipedia for Trench Warfare.
http://en.wikipedia.org/wiki/Trench_warfare
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One of the major disadvantages of trench warfare was diseases. Many soldiers contracted diseases from being in the trenches for so long. Another disadvantage was that since the passage ways were so narrow, moving bodies was very difficult.