In addition to its geographic proximity, Azerbaijan's historical and cultural ties to both Turkey and Iran provided fertile ground for both Sunni religious endowments and Shi'a mullahs to develop missionary activities.
Turkish and Iranian religious groups were later joined by Saudi Salafi missionaries already established in the Northern Caucasus. Their relatively simple interpretation of the Koran initially appealed to many Azerbaijanis amid the overall post-Soviet spiritual vacuum.
Today, Azerbaijan is the only former Soviet republic where the Turkish, Iranian, and Saudi brands of Islam are equally present, even though they do not enjoy the same level of influence.
Bayram Balci of the Istanbul-based French Institute of Anatolian Studies researches Turkish Islamic groups in Central Asia and the Caucasus. He told RFE/RL that being a predominantly Shi'a country, Turkic Azerbaijan is a special case in the former Soviet Union.
"What is extremely difficult for the Azerbaijanis is that, religiously, they are very close to the Iranians in that they profess the same duodeciman (Twelver Imam) Shi'ism," Balci said. "But culturally and historically they are much closer to Turkey. In any case, given the international context, they would like to get much closer to Turkey than to Iran, first because Turkey represents a sort of window on Europe, second because Turkey much better corresponds to the kind of state they see for themselves; i.e., a kind of secular state."
Balci argued that this explains in part why Azerbaijani authorities have been generally more lenient toward Turkish missionaries than toward Iranian mullahs.
Equally crucial, experts say, is the generally secular orientation of most Turkish groups.
Among Turkish religious activists operating in the country are followers of Osman Nuri Topbas, a group related to the Naqshbandiyah Sufi brotherhood that runs the Aziz Mahmud Hudayi endowment and carries out a lot charity work.
Also active is the movement led by Fethullah Gulen, a religious thinker who seeks to reconcile Muslim and modernity.
The fethullahci, as Gulen's followers are known, have spread across Azerbaijan and Central Asia, setting up a network of high schools and universities that attract children of the local elites.
A secular orientation and a proven ability to cultivate ties with Turkish political and business elites as well as with local governments have spared Gulen's followers many troubles in the former Soviet Union, except in Uzbekistan, where they were expelled at the end of the 1990s.
A distinctive feature of the fethullahci in Azerbaijan, Balci said, is that they have succeeded in forming local cadres who are present "in nearly all economic and social sectors" -- an achievement the French scholar ascribes partly to the comparatively greater openness of local authorities to religion.
"Azerbaijan offers believers greater opportunities to practice their religion and, at the same time, offers all these movements [comparatively] greater opportunities to proselytize," Balci said. "That makes it a lot easier for outsider groups to form local cadres and [become indigenous]."
Yet, the tolerance of the secular Azerbaijani government has its limits and not all representatives of Turkish Islam are equally welcome.
Authorities, in particular, have persistently denied registration to the Suleymanci, a radical group named after its founder Suleyman Hilmi Tunahan.
Raoul Motika teaches Islamic studies at Bochum University in Germany. He said the reception offered by Azerbaijani authorities to Turkish Islamic groups largely depends on their relations with Ankara's political establishment.
"The [further] these movements are away from Turkish state structures, the more problems they have in Azerbaijan. You have to differentiate between state structures, meaning the Turkish Directorate for Religious Affairs, which is officially very active in Azerbaijan," Motika said. "They have a theological faculty at Baku State University; they also have [high schools] and mosques. They are very, very active and [their activity] is all based on official agreements between the two countries. This is the first level. Then you have a second level, which is represented by semi-legal foundations from Turkey [like the Gulen or the Topbas movements], which are [also] active. Then you have these more or less illegal, or non-legalized, organizations like the Suleymanci. The first two layers -- the official and semiofficial -- are very active and are becoming very influential in Azerbaijan. All others are minor groups that are not important."
Azerbaijani authorities have developed various responses to stem the influence of foreign Islam.
In June 2001, then-President Heidar Aliyev ordered the creation of a state committee charged with reregistering all religious organizations throughout the country.
Although the government denied targeting any particular group, the move was largely perceived as designed to better control Iranian mullahs, Salafi Arab preachers, and Christian missionaries operating in the country.
The government also used the global war on terror that followed the 11 September 2001 attacks on the United States as a pretext to crack down on radical Arab Salafi -- commonly known as Wahabbi -- groups.
Yet, the Salafi creed of Islam remains present in the country, notably among the Sunni Muslim Lezghi minority that lives in northern areas close to Russia's autonomous republic of Daghestan.
Salafi groups also operate the Abu Baqr mosque in Baku and -- reportedly -- the so-called Lezghi mosque in the capital's Inner City.
Motika said doctrinal reasons also explain why Salafi proselytizers have not succeeded in spreading deeper roots in Azerbaijan.
"Although they are active [among] the Sunni part of the population, they face tough competition from the Turks. A large number of [Azerbaijan's] Sunni population -- how large, it is difficult to say -- has a Hanafi background, which means that they are closer to the Turkish brand of Sunni Islam than to the Arab [brand], which represents the Hanbali branch of the Islamic law that dominates in the Arab Peninsula," Motika said. "Wahabbi, or Salafi Islam, is an offspring of the Hanbali school of Islamic law. This means that the [influence of these Arab groups] is somehow limited, [although] the Lezghi community, or refugees from Chechnya, are sometimes quite open to such influences."
Hanafi and Hanbali are two of the four jurisprudential schools, or maddhab, that exist in Sunni Islam. Named after 9th-century Imam Abu Hanifa, the Hanafi school is considered to be the most open to modern ideas. By contrast, the Hanbali school is seen as the most conservative.
In addition to harassing radical Sunni groups or expelling influential Shi'a Iranian mullahs at times of tension with Tehran, authorities have been developing alternative responses to maintain control over Islam.
In the early 1990s, a state-sponsored theology university was set up with a view to replacing religious cadres formed in Uzbekistan in Soviet times.
Citing Turkey as an example, some officials would now like to see tightly monitored religious classes authorized in high schools. Pilot religion classes have been introduced in some teaching establishments. Yet, Motika said, the top political leadership remains strongly divided on the issue.
"Two factions within the ruling circles are fighting each other. One is represented by the Religious Affairs State Committee of [Rafiq] Aliyev, which wants to introduce religious classes in [regular] school curricula," Motika said. "But Education Minister [Misir Mardanov] is against [this] and, until now, Aliyev has not been very successful in that field."
A delicate combination of coercion and benevolence has helped Azerbaijani authorities avoid the emergence of political Islam.
In the late 1990s the government arrested leaders of the influential Iranian-oriented Islamic Party of Azerbaijan on charges of spying for Tehran. This group was succeeded by the Muslim Democratic Party, which enjoys much less popular support than its predecessor -- in part because of its alleged links with the government -- and is not seen as posing any serious threat to the secular regime.
Motika said he believes the initial religious wave that followed the demise of the Soviet Union has died away and that he does not see any group now "that could provide the nucleus of a revived political Islam." Yet, he warned this does not preclude any unpleasant development for local authorities in the future.
"The Azerbaijani state has been more or less successful in the short, or middle-term," Motika said. "But [in] the longer run, you never know."