not a cost-effective way to protect the society against enemy raids. William Rubinstein wrote "Pre-literate societies, even those organised in a relatively advanced way, were renowned for their studied cruelty.'" In Western Europe, since the late 18th century, more than 150 conflicts and about 600 battles have taken place. During the 20th century, war resulted in a dramatic intensification of the pace of social changes, and was a crucial catalyst for the emergence of the political Left as a force to be reckoned with. In 1947, in view of the rapidly increasingly destructive consequences of modern warfare, and with a particular concern for the consequences and costs of the newly developed atom bomb, Albert Einstein famously stated, "I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." Mao Zedong urged the socialist camp not to fear nuclear war with the United States since, even if "half of mankind died, the other half would remain while imperialism would be razed to the ground and the whole world would become socialist." A distinctive feature of war since 1945 is that combat has largely been a matter of civil wars and insurgencies. The major exceptions were the Korean War, the Indo-Pakistani War of 1971, the Iran–Iraq War, the Gulf War, the Eritrean–Ethiopian War, and the Russo-Ukrainian War. The Human Security Report 2005 documented a significant decline in the number and severity of armed conflicts since the end of the Cold War in the early 1990s. However, the evidence examined in the 2008 edition of the Center for International Development and Conflict Management's "Peace and Conflict" study indicated the overall decline in conflicts had stalled. Types of warfare Asymmetric warfare is a conflict between belligerents of drastically different levels of military capability or size. Biological warfare, or germ warfare, is the use of weaponized biological toxins or infectious agents such as bacteria, viruses, and fungi. Chemical warfare involves the use of weaponized chemicals in combat. Poison gas as a chemical weapon was principally used during World War I, and resulted in over a million estimated casualties, including more than 100,000 civilians. Cold warfare is an intense international rivalry without direct military conflict, but with a sustained threat of it, including high levels of military preparations, expenditures, and development, and may involve active conflicts by indirect means, such as economic warfare, political