And even in the way battles were fought, the changes brought about by World War I were numerous and significant. Trenches had, of course, been used in the past, but never on such a scale. For more than four years, millions of men lived in trenches, often surrounded by vast areas of mud, with little change in territorial gains for either side. Even when one of the opponents managed to gain a few kilometers of land, it often lasted only a short time, as they were soon forced to retreat to their previous positions. Losses on both sides were enormous, aided by a constantly developing "technology of death" (for example, the use of gases). At the same time, new weapons, such as tanks and airplanes, appeared, effectively replacing older forms of warfare. Cavalry units, for example, were unable to play the roles they once did, as they now had to face machine guns, new types of rifles, and barbed wire.