Answer:
Trench warfare was introduced in world war 1, while the united states and germans fought inside of trenches that were dug on both sides of enemy lines. The trenches often flooded and had many rodents such as rats, which would leave the trenches wet and in unsanitary conditions. This would often leave soldiers sick or with "trenchfoot" which can be attained by having wet socks or feet for long periods of time, and would often leave soldiers with amputated feet or legs. This resulted in the slow progress of the war as neither side could make large advances without being shot.