Over the course of the twentieth century, the way war was fought changed drastically. From the tactics to the weapons themselves, nothing stayed the same as two World Wars along with multiple others came and went. The first things to change were the tactics of warfare. Unlike in all previous wars in Europe, the two World Wars were not fought in straight lines, but rather in trenches dug out of the ground. In between either sides’ trenches would be an area filled with bomb craters, barbed wire, and toxic fog from various gases called “No Man’s Land”. Firebombing was another technique that was popular especially in World War II. Despite it not being militarily very effective, firebombing urban areas was extremely affective in decreasing citizen morale. Another major thing to change was the weaponry used. As tanks and machine guns were introduced in WWI, destruction of property increased. Mustard gas and other chemical weapons led to the casualty count rising drastically. Atom bombs in the latter stages of WWII led to mass destruction, radiation, and mutation in later generations in areas where they were used. At the onset of the 20th century, the greatest innovations in warfare were the fully automatic machine gun) and the breech-loading artillery piece. These two innovations were perfected early in the 20th century, and they changed the face of war. Casualties became astronomically higher. Tactics then changed to deal with these new killing capabilities. First came trench warfare to protect infantry. To counter trench warfare, chemical warfare was introduced, and armored fighting vehicles were introduced (tanks). Chemical warfare is interesting in that it's one of the extremely few examples we have of human beings reaching a consensus that it's a weapon that should not be used; its use has been rare since, and universally condemned.