After the disintegration of the Soviet Union, these weapons are readily available for purchase and also due cash – strapped defence, biotechnology, chemical, aerospace and nuclear industries; these ex Soviet Union countries are willing to sell their technology. There have been reports that scientific institutions and universities in Russia are willing to earn money by teaching the nuclear and WMD technology to students from foreign nations. are displaced WMD,
Motivations and emerging ideologies of terrorists,
Proliferation of technologies of mass destruction, and increased access to information and information technologies, which have increased the vulnerability to terrorist attack.
Terrorist groups are becoming more deadly and destructive by day. Terrorist incidents with potentially large-scale casualties have been on the rise as for these terrorist groups limited lethality does not hold any importance. Terrorism using WMD may be anticipated from Fanatical Religious Groups and cults, Islamic extremists, right-wing chauvinists, and loosely affiliated terrorists who lack the traditional concern or desire for group preservation. Proliferation of Weapons of Mass Destruction and technology from or to undesirable nations adds another dimension for its use.
Terrorists have access to means that was considered technologically complex and financially unaffordable. These terrorist groups are now more interested in acquiring technical knowhow of WMD. In an interview with ABC news correspondent John Miller, infamous terrorist Osama Bin Laden said, “To seek to possess the weapons that could counter those of the infidels is a religious duty. If I have indeed acquired these weapons, then this is an obligation I carried out and I thank God for enabling us to do that. And if I seek to acquire these weapons I am carrying out a duty. It would be a sin for Muslims not to try to possess the weapons that would prevent the infidels from inflicting harm on Muslims. But how we could use these weapons if we possess them is up to us.”  Because proliferation continues, constraints preventing the acquisition of NBC weapons may be surmounted by terrorist groups interested in causing mass destruction.
REASONS FOR USE OF WEAPONS OF MASS DESTRUCTION BY TERRORISTS
The attack on the Japanese train in 1995 came at a time when the terrorist strikes were on a decline. The number of attacks by terrorists had reduced since 1980s, but the potency of the attacks had increased due to increasing use of WMD and enhanced technology by these terrorist groups. Most of these terrorists groups had acquired state of art ordnance, timing mechanisms, and highly accurate Surface-to-Air missiles, as they could raise funds from various sources, including transnational fund raising infrastructure. What concerns are the increasing sophisticated operations mounted by these international terrorist groups. 
Another reason which increases fear in our mind is the emergence of terrorist groups motivated with religious fundamentalism. In a study by ‘Centre for the Study of Terrorism and Political Violence at St Andrews University’, in 1992, out of 48 terrorist groups, 11 were religious fundamentalists, including Christians, Sikhs, Jews, and Muslims. Any such group would like to have a chemical or biological weapon as compared to nuclear weapon as the earlier weapons are readily available and affordable. In addition, Chemical and Biological agents can be legitimately purchased, as they have dual use and the process of producing an agent is relatively simple.
WMD can be a potential weapon of choice by terrorists due to the following reasons:-
Quantity of WMD required would be much lesser as compared to conventional weapon to get desired effect.
Mass casualty achieved by using these weapons will have a huge impact on the psyche of the public.
As it has far reaching effect, any use of WMD may result in destabilising the government as the people would have lost faith in them.
Since it is hard to detect, a terrorist group can use it without being detected and also vacate that place. Lack of warning produces the maximum social and psychological disruption.
The devastation caused by this mass destruction can be large if it is burst over a large area using air compound.
Most of the after effects in case of a WMD use is still restricted to papers, and the general public is unaware of the unfolding of events in case a WMD is used, which may lead to helplessness, vulnerability and disequilibrium in society.
Purchase of material/ literature is relatively easier and can be sourced easily from the following sources :-
Lab/ commercial product.
Medical/University research facility.
Foreign Military Sources.
Nuclear Terrorism. Terrorist using nuclear weapon has since long been an anxiety, and its potential was underlined by Chechen incident of 1995. Nuclear terrorism can take many forms, including stealing or making nuclear weapon, either for blackmail or detonation, attack or seizure of a nuclear weapon site/ plant, procurement of fissile material, contamination or threat to contaminate targets using radioactive material. However, much of these threats would depend upon the technical expertise of a particular group, but its potential effect will always be high. 
In the past several terrorist groups talked about resorting to nuclear weapon , but were in general ,conservative in their tactics and in the level of violence they were willing to use to pursue their goal . Post cold war, the reason and capability to go nuclear has altered considerably. The newer groups have not only been more violent but also more innovative and willing to use new types of weaponry. Nuclear terrorism is more likely to be conducted for limited political rather financial gains and to require the resources of a group of psychologically normal individuals. Setting of a nuclear yield bomb would require that a group somehow acquire a weapon, possibly through state sponsorship, but more likely by stealing one, an extremely difficult proposition and a relatively unlikely one, given the presumed needs of terrorists or states for anonymity and complete security at such a delicate stage of their operations. The more likely option would be that they build a bomb; since this would require fissile material it would seem to preclude all but the most well off or state sponsored terrorists groups.
The design for a crude nuclear devise has been publicly available for past 25 years and relies on technology that, while challenging in 1940s, is almost certainly no longer so, particularly if terrorist are content with a crude nuclear weapon, of variable and uncertain yield. Furthermore, Plutonium oxide could also be used to for a crude nuclear device by placing a close to critical mass in a spherical container and surrounding it with large quantity of conventional high explosive. The simultaneous detonation would almost certainly result in significant energy from nuclear fission. Even if it did not work, the result will be an unpleasant radiological dispersal. A crude device could be built by a small group using the open literature, and without requiring the testing of the component, for a fraction of a million dollar. Adding to this threat is the emergence of low technology nuclear devices and radioactive material dispersal weapons, based on diverted or stolen material.
Chemical Weapon. A chemical threat is most likely as the literatures for making a chemical weapon is available in open market and the task can be achieved in a concealed covert laboratory. In case these groups are state- sponsored, then the problem is even simpler of acquiring these agents. The groups can also employ unemployed or underpaid scientists to work for them in preparing a chemical weapon. The Russian chemical weapon storage facility lacks security, so that can be a target for these groups. The lethality of toxic chemical release in atmosphere can be judged by the number of deaths in the accidental release of methyl isocyanate in the Union Carbide factory at Bhopal in 1984. As per CIA, roughly one tonne of chemical agent is required to contaminate one square mile area, provided it is disseminated in the correct manner. The chemical attack by the terrorist could be contaminating the air disseminating system of train, aircraft or public transport, poisoning food or contaminating the water supply at its source (a difficult proposition compared to the amount of water handled by the municipality). Chemical agent can be an attractive option for terrorists, if they wished to inflict large number of causalities, cause panic, and disrupt society. It can be used against military, political or symbolic targets, as an instrument of revenge.
The use of chemical weapon against a state, will give the terrorist initiative, shock the target audience, and undermine the audience’s confidence in the government. The psychological effect of a chemical weapon can be overlooked as it will revive the effects of poisonous gas.
A Chemical agent has certain advantage in its use. They have long been considered “the poor man’s atomic bomb” due to their relative low cost and ease of manufacture. The other aspect which makes chemical weapons such an appropriate weapon for a terrorist is its terrifying nature. Ever since its first use, they have been criticised and ridiculed by civilians and soldiers alike. They’ve are unconventional, uncivilized, and even gruesome, adjectives which are synonymous to terrorists. In general terrorists thrive off the shock factor of their activities and chemical welfare exhibits a high degree of shock factor. And finally, inflicting mass casualty is the main reason for its use. These weapons are extremely cost effective and 40 times more weight effective than conventional explosive weapons .
Biological Weapon. If a biological weapon is effectively dispersed, then even a small amount of such agents can cause devastation which would be much larger than chemical, conventional or nuclear weapon. Production of a biological weapon is much simpler, cheaper, and can be concealed in a much better way than its counterparts. If it is dispersed as an aerosol, than it will be tasteless, odourless, invisible, silent, and the first sign of it would be only when people start falling ill or dying. Use of biological weapon cannot be detected, as it may pass of as a natural disaster. Like chemical weapon, biological agents can be cultured anywhere, but in a much simpler manner. The lethality of a BW depends on the dispensing technique. Terrorists may resort to small scale operations, as a large scale operation would require sophisticated delivery system. Even a partially successful aerosol attack in a in a crowded place, can produce a delayed but heavy causality.
The terrorists can use biological weapon due to the following factors:-
Experts from biotechnology or pharmacological industry can be hired without much surveillance.
Greed may be a factor to motivate certain group.
Quantity of agents required to effect large causality is very small in volume and weight.
A biotech weapon can pass through most of the security barriers.
Delivering biological agents does not require sophisticated means.
As on date, no defensive technique is available to detect its use.
In case a biological weapon is used in a heavily populated area, the casualties will be very high. In addition, the doctors would have to deal with a large number of patients who were infected at the same time. In case of India, any such attack may lead to epidemic or a pandemic, as such a large number of cases may not get treatment due to lack of infrastructure.
WMD VS CONVENTIONAL THREAT
As on date, states have been reluctant to use Weapons of mass Destruction because of political retaliation, and similarly terrorists have also not used WMD. Even states sponsoring terrorism have not handed over WMDs to these groups, as any retaliation by the world might be against the state rather than against the terrorists. Terrorists on the other hand, are supposed to be of the opinion that any use of WMD by them, might turn the tide against them.
The prospect of use of WMD has increased many fold due to increased availability of technology, expertise, material and dual use technology. This is complemented by the changing nature of international terrorism, with diversified motives, and capacity to carry out coordinated and complex attacks. The terrorist groups are likely carryout WMD attacks at a low scale, however those who have state backing can go full bore in its use. Even a failure of these low scale attack, can cause substantial panic amongst the population.
The motive of violence of terrorists, with no hold bar motive is realistic. The terrorists in trying to achieve their motive, overlook the law customarily take the edge off the excesses of armed conflict and are involved in war of attrition to show their presence. As per the terrorists’ belief, coexistence and compromise with the state is not possible, and hence all their efforts are directed towards violence.
These terrorist outfits, in order to successfully employ a WMD, needs to be proficient in using these deadly weapons. In addition, they also need to overcome moral constraints. In the end, these groups would endeavour to use the WMD despite formidable costs, as money for these outfits is not a constraint. Though very few group can meet the capability of acquiring and using WMD, the group which are most likely to use these weapons would be the religious fanatic groups, that are seeking revenge or those attracted to violence for their own sake. The likely reasons for the use of WMD by terrorists include religion, millennial, right-wing, and ethnically based terrorism.
Most of the terrorists groups of today have cropped up with religious background and religion are going to be driving most of the terrorist outfits in future. The biggest example of this is the Al Qaeda, JeM, and other Islamic fundamentalist groups. These groups in the belief that they are doing a service on Gods instruction are most likely to use these weapons. Religious terrorist Osama bin Laden expressed his sincere desire to obtaining chemical and nuclear weapons for the purposes of removing the non desired States before eventually creating a worldwide Muslim state. 
Terrorists who are religiously motivated may involve themselves in more intense acts of violence causing higher death tolls and elevated levels of destruction. These religiously motivated terrorists believe that “Holy Fighters” will be rewarded in the rebirth if martyred in pursuit of their religious goals. These religious terrorists believe that the act of violence by them is directly approved and ordered by the god. Terrorism then becomes an all-encompassing and superior duty that usually finds its perpetrators free from the political, moral, or practical constraints that may affect other terrorists. Moreover, religious terrorists may be more inclined to seek the total devastation or removal of their adversary and would view WMD terrorism as morally justified and appropriate for their purpose.
Religious terrorism refers to those groups or acts that are primarily religious in nature, but may have a political agenda as well. Some authors contend religious terrorism as being the most important defining characteristic of terrorist activity today. Terrorism motivated by religious differences has received a great deal of attention in the last four decades and is considered by many to be a likely source of future terrorism using WMD. Traditional terrorist motivations have begun to shift from “ideology and separatism to ethnic animosity and religious fanaticism motives that lend themselves to wide scale atrocities.”
The first ‘modern’ religious terrorist groups appeared in 1980 as a result of the Islamic revolution in Iran the previous year. “Only two of the sixty four groups’ active in 1980 could have been classified as predominantly religious in character and motivation. These were the Iranian-backed Shia organization al-Dawa and the Committee for Safeguarding the Islamic Revolution.” Since then, the number of religious terrorist groups has increased greatly.
NUCLEAR TERRORISM BY RELIGIOUS FUNDAMENTALISTS
Technically, assembling enough fissile material for even a crude nuclear device, and the amounts needed vary inversely with sophistication, would be very difficult and extremely expensive for a terrorist organisation. The theoretical knowledge and practical skills required to design and build a nuclear weapon are of a high order, while setting up, equipping and successfully operating an undetectable clandestine weapons laboratory would be expensive and difficult, even for the best-funded terrorist organisation. No state possessing nuclear weapons would be likely to hand over nuclear weapons to terrorists unless they were acting as mercenary agents of the state itself. The threat of nuclear retaliation, even if the possibility of tracing the weapons back to its source were thought to be low, it should be enough to deter any rational state from using a nuclear weapon against another nuclear-weapon state, or a country under the protection of one. Aside from the technical issue, there remains the question of whether terrorists would use nuclear weapons if they somehow managed to obtain them. Terrorists have different goals, histories, leaderships, cultural roots, political contexts, moral beliefs, constraints and strategies. Of these types, apocalyptic religious or cultist terrorists are the most dangerous, from a motivational and strategic viewpoint.
The next most dangerous form of terrorism is the religiously inspired transnational variety uniquely exemplified by the Al-Qaeda. The group has a narrower and better defined political goal when compared to a cult; it has very few characteristics of a purely religious group. Its struggle is against the world of real political actors. The outcomes it desires are equally concrete and to the extent that it engages in strategic thinking, it does so in response to the configurations and dynamics of power in the real world. Al-Qaeda can easily differentiate, on ethnic grounds, its target populations from those on whose behalf the organisation claims to be acting. Its potential targets for nuclear terrorism are far from its heartlands. Terrorists are not known for their any tactical innovation. The traditional roots of terrorism, hostage-taking, bombings, shoot-and-run attacks, have now expanded to include the suicide bombers. Terrorists in general probably share the same ignorance and fear of WMD prevalent in the broader population, and probably see little reason to turn to unknown, possibly unpredictable and certainly dangerous substances and techniques, when the older tactics have proved themselves to be simple, reliable and cheap.