Wahhabism is an offshoot of the Hanbali school that arose as a reformist movement in the 18th century and existed in opposition to all of mainstream Islam, both Sunni and Shiite. However, due to its finding favor with the House of Saud, and the enlargement of the political and economic power of the latter after World War I, it became accepted as the official interpretation of Islam in Saudi Arabia.
Wahhabism is fundamentalist in its tendency due to a combination of factors, namely its chosen response to the invasion of the Islamic world by the West, and the reductionistic view of Shariah that it retained from its Hanbali origins, combined with a strictly literal reading of the Koran. In this way it has tendencies that resemble those of Christian Protestantism, being highly exoteric and exclusivist in its handling of doctrine.
Given the prevalence of Wahhabism in Saudi Arabia, and the fact that Medina and Mecca are also situated there, one can easily get the impression that Wahhabism is synonymous with mainstream Islam, but most of the Sunni world cannot in any way be described as Wahhabi, much less can we identify this movement with Shiism, which has always opposed it.
The problem is made a bit more convoluted by the fact that not many ‘Wahhabis’ would actually call themselves that, since it more often appears as a derogatory labels used by their opponents. Instead they would more often choose to identify as ‘Salafi’, which is not so much a maddhab or school of thought within fiqh as it is a general reformist movement, of which the Wahhabis would form a certain type. We could say that Wahhabis could be considered as an overlap between Hanbalism, which pertains to fiqh or jurisprudence, and Salafism, which is a more general reforming movement opposed to speculation in theology (kalam), which for its part could be set against Ashari kalam, which is the more prevalent Sunni view on theology and more open to speculation.
We hesitate to delve into this in much greater detail, at the risk of speaking outside our competence, and what has been presented above already feels like an oversimplification. We will, however, return to certain related points later on in our discussion, particularly with regard to reactions against Western encroachment and Islamic ‘fundamentalism’.