When news broke of the double bombings in east Africa, it was not just western security services that suddenly began to take Osama bin Laden and his al-Qaida group seriously. Many within the Islamist militant movement were impressed.

Until that moment, Bin Laden had been seen as something of a dilettante, a rich young man who had never experienced first-hand the tough, day-to-day battle in streets, safe houses and cells across the Middle East that Islamist militancy had meant until that point.

The Saudi-born organiser and propagandist had of course seen action during the 1979-89 war against the Soviets in Afghanistan, but then so had thousands of other young men from across the Islamic world. When many veterans of that fight went on to spearhead brutal campaigns of violence in countries such as Algeria, Egypt, Jordan or Pakistan, Bin Laden went home to Jeddah and then into a relatively comfortable, safe exile in an Islamist-friendly Sudan.

But the east African attacks “made a lot of people sit up and take notice”, said one Libyan veteran activist four years after the strikes.

Founded in Pakistan in the late 1980s, al-Qaida had long been a marginal group without significant resources, manpower or an obvious strategy. The highest profile terrorist attacks of the 1990s – on the World Trade Center in 1993, in Saudi Arabia and in Pakistan – had been the work of other groups.

Bin Laden had been tangentially linked to a strike on a hotel used by US servicemen in Yemen, and to violence in Somalia, but nothing else. His tirades against the west and calls for every Muslim to attack US citizens and interests wherever they could be found, issued after his return to Afghanistan in 1996, were controversial even within his own organisation, as one forthcoming memoir by a militant makes clear.

All that changed in 1998 with the East African bombings. In their aftermath, the Clinton administration launched cruise missiles on poorly selected targets in Afghanistan and Sudan.

The ill-judged response to the bombings made the Taliban, who had been initially wary of the extremist Arabs living in territory they had recently conquered, more sympathetic towards Bin Laden and his global agenda. This removed a key obstacle to further, more ambitious attacks.

The strikes, especially when followed by the attack on the guided-missile destroyer USS Cole off Yemen in 2000, also allowed Bin Laden and his group to claim leadership of the fragmented militant movement. They became a favourite destination for volunteers, donations and, crucially, groups from all over the Islamic world seeking assistance with projects of their own. This laid the basis for the network of networks – the affiliates – that are still so important today.

The East African bombings also popularised the new conception of a conflict no longer limited by territorial boundaries. In the new globalised world of the late 1990s, with its satellite TV news channels, cheap flights, relatively open borders and nascent internet, this broader vision simply made more sense to aspirant militants from Morocco to Malaysia than the parochial vision of other groups active at the time.

The local struggles they had waged had all failed miserably, crushed by repressive authorities or undermined by flagging public support amid chaotic and apparently indiscriminate violence against other Muslims.

From the embassies in Africa, theoretically US sovereign territory, to an attack on a US warship at sea, to bombing the US in the “homeland” itself was an inevitable progression. We are still living with the consequences today.