A global network of terrorist and militia groups. A desire to establish an Islamic caliphate and expel non-Muslims from Muslim lands.

A willingness and preparation to execute "spectacular" attacks in Western countries. Growing strength, capability and influence.

No, this not the Islamic State (IS) group, the terrorist group that has dominated the headlines since it declared a caliphate in Iraq and Syria in 2014.

This is the group that spawned IS: Al Qaeda.

You might have forgotten about Al Qaeda after the US killed its leader, Osama bin Laden, in Pakistan in 2011.

The group has been relatively quiet since then, taking a deliberate strategy of avoiding major attacks and especially not attacking Western targets.

"I believe that Al Qaeda has deliberately eschewed international terrorist operations, especially 'spectaculars', precisely to avoid attracting attention to itself," said Bruce Hoffman, a researcher on global terrorism at Georgetown University.

"Al Qaeda … is certainly still alive.

"I would argue [the group] has been quietly and very patiently rebuilding and continuing to marshal its resources to carry on the struggle that Osama bin Laden declared more than two decades ago."

That struggle is an ideological and physical battle against "unbelievers", or enemies of Islam.

The group's leader, Ayman al-Zawahiri, said its focus is on attacking the US, which it calls "the head of the snake," and the "first enemy of Muslims around the globe".

Almost 3,000 people were killed in the attacks by Al Qaeda on the World Trade Centre and the Pentagon. ( Reuters: Sara K. Schwittek )

This long-held aim led Al Qaeda to carry out the September 11, 2001 attacks on the World Trade Centre in New York and the Pentagon in Washington, DC — the single deadliest terrorist attack in history.

Al Qaeda's eventual goal is still the establishment of a caliphate from Jerusalem to Spain.

It's a very similar goal to that of IS, partly because IS began as Al Qaeda in Iraq before an ideological dispute saw it split from the group in 2013.

Since then, Al Qaeda has allowed Islamic State to be the focus of attention and attacks from a coalition of Western countries, including Australia, that saw IS lose control of all its territorial caliphate in March of this year.

Islamic State was a splinter group of Al Qaeda, formed after an ideological dispute in 2013. ( Reuters: Khalil Ashawi )

While IS was being bombed by coalition planes and losing a ground campaign in Iraq and Syria, Al Qaeda was quietly strengthening its network of affiliates across the world, including in Indonesia.

"Al Qaeda aspires to fill the vacuum that has been created by the defeat of Islamic State," Professor Hoffman said.

"All this behind-the-scenes activity suggests ongoing preparations, at some point, when Ayman al Zawahiri, Al Qaeda's leader, thinks the time is right to resume operations."

Analysts fear Al Qaeda will shift focus back to the West

Al Qaeda affiliate groups have popped up in dozens of places, including the Ansar Ghazwat-ul-Hind group in Kashmir. ( Reuters: Danish Ismail )

Although Al Qaeda has avoided targets in the West in recent years, the organisation remains a threat for Western governments.

In June, police in Spain arrested 10 men for funnelling money to Al Qaeda-linked groups in Syria.

And last year, Britain's security minister warned Al Qaeda was developing technology to bring down planes and target airports.

"The aviation threat is real. Al Qaeda are resurgent. They have reorganised," Ben Wallace told the Sunday Times.

"They are pushing more and more plots towards Europe and have become familiar with new methods, and still aspire to aviation attacks," he said.

While Al Qaeda have been quiet, they have not been idle. At some point, observers warn, they will attack the West again.

"They're certainly in more places than they've ever been. They were [previously] in a handful of countries," Professor Hoffman said.

Professor Hoffman estimates Al Qaeda is present in at least two dozen countries, with up to 40,000 armed fighters.

"Al Qaeda is building an increasingly robust local presence that it hopes to eventually resurrect its international terrorist capabilities from," he said.

Al Qaeda uses franchise model to spread its reach

The Al Qaeda linked group Al Shabaab launched an attack on a Kenyan hotel earlier this year. ( Reuters: Baz Ratner )

Al Qaeda's strongest local affiliates include the Somalian terrorist group Al Shabaab, which attacked a hotel complex in the Kenyan capital Nairobi in January.

It said the attack was in retaliation for the United States moving its embassy to Jerusalem.

Matt Bryden, a political analyst and former head of the UN monitoring group on Somalia and Eritrea, said Al Shabaab was trying to become more of a transnational terrorist organisation.

"Al Shabaab itself is binding these threads of jihadism more and more closely together — its own Somali-style brand and the kind of extremism that exists elsewhere in the region," he said.

Al Shabaab has focused more on Somalia, but experts say the militant group seems to be branching out. ( Reuters: Feisal Omar )

"That should worry anyone in this part of the world, because the last time we saw this was when Al Qaeda was very active here.

"When these things come together it's very dangerous, and I think it's quite possible that's what we're seeing now."

In 1998, Al Qaeda bombed the US embassies in Kenya and Tanzania, killing 224 people.

In a single day in 2002, the group attempted to shoot down an Israeli airliner and bombed a tourist resort in Mombasa, killing 13.

Al Qaeda reportedly has a strong presence in Afghanistan and affiliates in north Africa, and is renewing its affiliations with militant groups in Syria's last rebel-held province.

It has also been active in the ongoing war in Yemen, where its offshoot Al Qaeda in the Arabian Peninsula (AQAP) has gained combat and governance experience.

Al Qaeda in the Arabian Peninsula is locked in a civil war with the Government for control of Yemen. ( Reuters: Mohamed al-Sayaghi )

It even administered a city for almost a year before being ousted by forces led by the United Arab Emirates.

Al Qaeda assumed control of municipal services, police, and even disaster relief following a cyclone, according to senior Yemen analyst with International Crisis Group, Peter Salisbury.

"They did everything they could to demonstrate they were the more capable governance actor," he said.

"They're really competing on a more political level with internationally recognised governments … to show that they were the better choice for Yemenis."

But Mr Salisbury said Al Qaeda in Yemen had also become a more divided and looser organisation than one purely focused on attacking international targets for ideological reasons.

"They've become much more focused on integrating with local spheres and much less focused on the brand-name, big-ticket attacks," he said.