A Weapon of Mass Destruction (often abbreviated as WMD) is an instrument of warfare that is purposely designed to cause loss of human (or, as the case may be, other sentients') lives in the scale that makes even the threat of using one a viable tool in the international politics (or its equivalent).
WMDs tend to be indiscriminate, doing as much harm to your own side as to you your enemy's. For that reason, the willingness to actually use WMDs is generally seen as the mark of a madman. Civilized nations are more likely to agree to conventions prohibiting the use of WMDs than more controlled weapons. Of course, that doesn't stop those nations from stockpiling WMDs, "just in case…"
In general, these things work at least as well as a deterrent as a weapon…
- Biological Weapons are based on usage of pathogenic microorganisms like bacterias and viruses. Even though germ theory wasn't developed until mid-19th century, the intuitive understanding of the principles has been with humanity almost as long as civilization has existed; it makes biological weapons arguably the oldest WMD in existence and application. Biological weapons can either attack humans or, more insidiously, vital food crops or other infrastructure targets.
- Chemical Weapons utilizes toxic agents (either synthesized or naturally occurring) to destroy the enemy. First notifiable use of such happened during WW11, and humanity quickly came to understand both the basic methods of protection against them the extent (and limits) of their destructive potential. It it worth noting that all of the sides of WWII had sizable chemical arsenals, but none of them dared to put chemical weapons to use, no matter how dire situation was2…
- Nuclear Weapons unleashes the immense energy of uncontrolled fission or fusion upon your adversary. There have only been two "live fires" to date, neither of which caused as much devastation as 'carpet bombings' of late WWII, but were far more spectacular. However, technology advanced quickly and their destructive potential was one of the factors that defined the Cold War period of Earth's history.
- Radiological Weapons turns one of the side-effects of the nuclear warfare — the radioactive poisoning of the surroundings — into its main effect by dispersing radioactive material with a conventional explosive. Although less physically destructive a radiological bomb is a lot easier to source and build and has the potential to leave a legacy of extremely long lived contamination. In the long term, they may be most hazardous of the WMDs.
- Ecological Weapons deliberately destroy the environment in a given area - this category would include radiological weapons, biologicals and some chemical weapons as well as some other, more theoretical types. On a small scale, sowing your opponents fields with salt and posioning the wells might qualify … on a larger scale this has the potential to add a whole new scale of atrocity to the field of war cimes.
Fictional or Hypothetical WMDs
- Antimatter weapons - Potentially tiny explosives with a much higher yield than a nuke. It's all about the antimatter.
- Gates - mostly fantasy universes only, opening a permanent connection to Hell, or some other hostile plane
- Geophysical Weapons causes disasters like earthquakes, volcano eruptions or ozone-layer holes happen on (or above) the enemy territory.
- Magic can fill this slot as well as it can any other. Typically the magic in question will be suitably epic … if it isn't you have to wonder why there's anyone left standing.
- Meteorological Weapons are, simply and plainly, something that creates the bad weather on the scale of Great Deluge.
- Mind Control Weapons - such as Mind Control Lasers installed on government satellites
- Orbital Kinetic Weapons
- Probabilistic Weapons
- Self-Replicating Nanoweapons, or grey-goo consists from billions of tiny mechanisms capable to produce their exact copies from whatever materials the have. Given enough energy and matter, one usage of such weapon might render all of planet's surface uninhabitable. This may not even start life as a weapon…
- Spirit Nuke - either a device generating a WMD level magical attack or a conventional nuclear weapon optimised for its supernatural output.
- Stardrives - A large spaceship with an engine capable of reaching a significant percentage of the speed of light would be able to make one heck of a kamikaze attack.
- Temporal Weapons
- Zombies, of the infectious variety … may be biological, magical or some combination of the two.
- The Mountain Hammer for a complicated use of a fantasy WMD as a McGuffin.
Game and Story Use
- It's a classic plot that never gets old: A madman has developed or gotten hold of a WMD and is threatening to unleash it on the population! Only your heroes can stop him!
- Our heroes have stolen (or otherwise accquired) a WMD that they cannot safely destroy (or want to sell) … can they get it to someone trustworthy/the buyer without getting robbed and/or killed?
- This could involve them literally stealing the bomb.
- It's wartime and we've identified one of the enemy's manufacturing sites for a WMD that might cost us the war. There's no way we can hit it with conventional troops but a small team of specialists might be able to get in there.
- It's wartime and we have a secret weapon that could win us the war - but we are worried that enemy spies have escaped with crucial details/will infiltrate and destroy it. Stop them or all is lost!
- It's nearly wartime, but the imminent enemy have a weapon that could destroy us utterly. It's too widespread to destroy - we need to copy it so that we can deter them from using theirs.