By Mark Duff BBC religious affairs reporter

The devastating nightclub bombing on the Indonesian holiday island of Bali has again focused attention on the political power of Islam to motivate people to acts of extreme violence.

It has also raised questions about the specific characteristics of Islam in the world's most populous Muslim country.

A quick glance at a map of Indonesia suggests the key to its special blend of Islam. It is huge - the world's biggest island chain.

Radical Muslims do not command wide support

It is at the crossroads of the Pacific and Indian oceans. And it has been on trading routes for as long as mankind has sailed the seven seas.

Geography, history and trade - those three factors have combined over the centuries to create a brand of Islam that is unique to Indonesia.

It is a brand that acknowledges the great tradition of religious belief out of which Islam emerged. And it is one that is enshrined in the country's constitution.

Geography

Indonesia's sheer size - it spans 5,000 kilometres - means there are significant variations within Islam from one part of the country to another.

Muslim traders from the Arabian cradle of Islam brought their faith to Indonesia. But Islam was not always treading on virgin territory - and the Indonesian Islam of today reflects that historical and demographic reality.

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The earliest Indonesians were animists. They practised ancestor and spirit worship. Then, for about 1,500 years after Christ, the Hindu and Buddhist faiths spread from the Indian sub-continent to the western part of Indonesia.

Some pockets of Indonesia remain predominantly Hindu - most notably Bali. But the Hindu kingdoms never reached the eastern islands.

Islam reached Indonesia from India. It was the mainstream Sunni variant of Islam that was found originally in Arabia and is today the faith of about one billion believers worldwide.

But the Islam of Indonesia was also influenced by Sufi holy men - devout Muslim mystics renowned for the beauty of their music and poetry, and for internalising the focus of their spiritual odyssey rather than seeking to impose their faith on the external political realities around them.

That influence, too, was a force for moderation.

In time, Islam spread throughout the archipelago and up into the southern Philippines. For the most part, it was a peaceful conversion - unlike the Arab and Turkish conversions made at the point of a sword.

Its final form in any place depended on the inherited tradition. Where Islam overlay Hinduism, or the ancient belief systems that pre-dated it, as in the west of the archipelago, it differed from the form that emerged further east, which had been untouched by Hinduism.

But the same geography that helped foster moderation and tolerance now has more sinister connotations. An archipelago of thousands of islands is ideal territory for guerrilla groups: hunting them is like looking for the proverbial needle in a haystack.

History and politics

Indonesian Islam came into its own politically with the battle against colonialism. In Sulawesi province, and later in Aceh, Islam became a rallying point for resistance to the Dutch colonialists. Militant Islam is nothing new in Indonesia.

Politically, too, though, Islam is part of a rich multi-cultural mix. The very symbol of Indonesia, the golden Garuda eagle, reflects this. It was originally a Hindu symbol.

Vice-President Hamzah Haz leads the largest Muslim party

The national motto is "unity in diversity".

The founding principles of Indonesia, the Pancasila, include a belief in God. But beyond this, religious tolerance is seen as the cornerstone of relations between different faiths - even though almost 90% of Indonesians are Muslim.

Moderation is therefore built into the country's constitutional framework.

This is not to overlook the impact of decades of repression during which the armed forces clamped down on Islamic militancy. The authoritarian rule of the military provided the stability on which economic growth took place. It provided security and relative peace.

But it also entailed severe restrictions on personal freedom of expression.

The advent of democracy, coupled with the impact of the South-East Asian economic collapse in 1997, and the arrival of a tough new breed of Middle Eastern Islamic preachers, sowed the seeds of the current challenge to Indonesia's traditionally moderate form of Islam.

Economic stress created an underclass of impoverished, alienated people. Democracy brought freedom of speech, and a new generation of Islamic politicians used their newfound freedom to tap into this rich seam of anger and resentment.

More radical, purist Islamic preachers started to arrive too, from the Middle East - their interpretation of Islam very different from that of most indigenous Muslims and their moderate politicians.

Megawati's challenge

Indonesian President Megawati Sukarnoputri is walking a tightrope in the aftermath of the Bali bombing. How does she tackle the threat of a minority of radical Islamists, without alienating the overwhelming majority of moderate Muslims?

The scale of that challenge became clear when the country's two biggest moderate Muslim groups criticised the detention in hospital of Abu Bakar Ba'asyir, the radical cleric alleged to be the spiritual leader of Jemaah Islamiah (JI).

The US has branded JI a terrorist organisation. And while Indonesia has not named it as a suspect in the Bali bomb, it has admitted the group has links to al-Qaeda.

JI has an avowedly radical Islamist agenda - one which includes a vision of an Islamic theocracy across much of South-East Asia.

Since 11 September Indonesia has been under pressure to crack down on its home-grown Islamic militants.

The leaders of two groups - Laskar Jihad and the Islamic Defenders Front - have been detained and are facing trial. There have been small-scale protests by their supporters, but nothing - yet - to suggest there will be a popular backlash.

The Indonesian authorities say Mr Ba'asyir is a suspect in a series of bombings in December 2000, and alleged plots, including an assassination attempt on Megawati before she became president.

He is not being detained in connection with the Bali bombings.

But many Indonesians are sceptical. And while the vast majority of Indonesians are undoubtedly hostile to the radical agenda of some Islamists, analysts are warning that too harsh a crackdown could threaten those very values of free speech and tolerance that underpin Indonesian democracy.

And there is a more immediate political danger for Megawati - the risk that a crackdown could result in the break-up of her fragile coalition government, some members of which are sympathetic to the Islamic fundamentalist cause.