In the modern setting, nations that declare war have caused damage to the environment.
The trend of modern warfare causing calamitous impact to the environment can be seen in World War I and World War II. The first "modern war," World War I was particularly destructive to the environment. The premise of stopping adversarial aggression results in environmental destruction. This can be seen in how nations during World War I engaged in direct opposition with one another devoid of any care regarding the environmental impact of such aggression: "Germany sank an Allied ship containing a million pounds of mustard gas. The slowly leaking gas is expected to pollute surrounding waters for the next 400 years." The use of the land mines and chemical warfare in the World War I years poisoned the earth, vegetation, and the air for decades to follow. This same condition of environmental blight was evident in World War II, culminating in the dropping of the atomic bomb on Nagasaki and Hiroshima. In these actions, the conflict between wanting to end war and creating environmental havoc presented itself. Nations chose to obliterate the other as opposed to recognizing the damage done to a shared environment. In this light, one of the primary conflicts regarding the environmental impacts of war is that nations so driven to defeat the other often do so at the cost of environmental health and awareness.
Sadly enough, this same legacy has been present in wars to date. In the Vietnam War, the extensive bombing from military campaigns such as "Operation Rolling Thunder" destroyed much of the shore line, causing immense flooding and homelessness amongst the Vietnamese population. The use of chemical warfare such as Napalm and Agent Orange destroyed much in way of forests and natural habitats, as well as causing health problems and death within human beings. The toxicity of such chemicals along with their use on a mass scale set back the South East Asian region for decades. Once again, the conflict between wanting to destroy "the other" collided with a sense of environmental respect, with the former overcoming the latter.
In the most recent war endeavors in Afghanistan and Iraq, the sad legacy of favoring obliteration over environmental conservation has become the result of the conflict between both ends. The use of air bombing missions, so- called "smart bombs," on such a protracted level was undertaken with "approximately 340 tons of missiles containing depleted uranium (DU)." The consequence of this has been water and soil pollution from which the region will not recover for some significant period of time:
In addition, the allied bombing campaign of a variety of toxics-releasing sites such as ammunition depots, and the intentional setting of oil fires by Saddam Hussein during the invasion of Iraq in 2003 led to air, soil, and water pollution.
The presence of toxic dust, remnant of over a decade of war- based conflict, has caused airborne diseases and conditions ranging from respiratory illness to excessive cancerous cell growth. The deforestation that has resulted from war has impacted the photosynthetic production of Carbon Dixoide and release of Oxygen in these areas. The intense production of greenhouse gases has also been present in the most recent wars in Afghanistan and Iraq: "...the U.S. military used 1.2 million barrels of oil in Iraq in just one month of 2008." War vehicles rarely have to pass emissions tests, so this has contributed to more environmental destruction. The conflict between nations professing to be environmentally sound and clinging to military aggression presents itself. Care for the environment has consistently been treated as a non- existent entity in the pursuit of war agendas.
UN Secretary General Ban Ki- Moon has suggested that this lack of balance must be counteracted. He argues that the modern comprehension of war must be taken into account with environmental sensitivity:
The environment has long been a silent casualty of war and armed conflict. From the contamination of land and the destruction of forests to the plunder of natural resources and the collapse of management systems, the environmental consequences of war are often widespread and devastating.
The paradigm that nations have offered in the conflicts between waging war and concern for the environment has been one of national security. In order to evade a potential conflict between both equally desirable, but ultimately incompatible courses of action, nations have suggested that national security should outweigh all other interests. In this light, it is fair to question how valuable the end result of preserving national security is if the damage to the environment is so pervasive as it has been in modern warfare.
I think, in this case, you could consider the environmental effects of both World War II and the Vietnam War. For instance, World War II's use of nuclear weapons in Japan affected the remaining people in the area to suffer in relation to health. Those who survived were plagued by physical and mental disabilities. Many experienced birth defects even years after the incident. During the Vietnam War, U.S. soldiers used agent orange to destroy vegetation regularly. Vietnamese citizens were bombarded by its use experiencing health defects and death as well. A lot of soldiers were also directly affected by this substance.