Nuclear biological and chemical
Nuclear, biological and chemical weapons, often considered unconventional "mass destruction" means of warfare, are primarily used for terrain denial due to their persistent nature. No one from either side in a war can remain in an area contaminated by radiation, disease vectors or chemical weapons (such as mustard gas or nerve agents). Nuclear weapons have only been used once, when the United States bombed Japan to end World War II. Chemical weapons were widely used by both sides during the First World War and have been increasingly sought after by dictators in modern times due to the relatively simple technology required to produce them and due to their low cost. An example of their use in more modern times would be Saddam Hussein's decision to bomb Kurdish enclaves in his own nation the late 1980s.
International treaties ban such weapons, but as not all countries will recognize such treaties, the United States military is prepared with countermeasures for them.