Disadvantages of war include death and injury of large numbers of people, loss of economic resources, destruction of the environment, loss of productivity and lasting damage to military personnel. The most costly war in terms of loss of life was World War II, with 84 million people killed.
The conflicts in Iran and Afghanistan in the early 21st century are thought to be the most financially expensive wars in history, with a cost to the United States of $6 trillion. The conflicts also have destroyed forests and contaminated water supplies in the Middle East. Military personnel who see combat in war often suffer lasting health problems, including physical injuries and mental issues such as depression and post-traumatic stress disorder.