In the aftermath of the attack on Garissa University in Kenya, in which at least 147 people were killed, the world's attention is once again on the Somali Islamist group al-Shabaab.

The militants have grabbed the world's attention countless times before, but how much do we know about their roots and goals?

Here are some key points to understanding Somali militant group al-Shabaab.

The roots of al-Shabaab

Al-Shabaab means 'The Youth' in Arabic, and the group grew as a splinter group from previous Islamist movements within Somalia.

In 2006, the Islamic Courts Union (ICU) took control of Somalia, wresting control from the warlords and implementing a system of Sharia law to impose order on what had for 15 years been the world’s most lawless state.

The rise of the Islamic ruling power led to an invasion by neighbouring Ethiopia, a Christian nation, backed by the US. A brutal invasion soon resulted in the end of the ICU but left a vacuum, which was then filled by the few remaining hard-line factions – with al-Shabaab chief among them.

The Ethiopian invasion was the making of the current al-Shabaab, as the group radicalised and grew from a base of a few hundred fighters, to become a fully fledged guerilla army of thousands. In February 2008 the US State Department designated the group a terrorist organisation.

The battle for Somalia

• 2008-2011

In the period from 2008 to 2011 the group thrived, taking control of large swathes of Somalia and almost the entire capital city, Mogadishu. At one point al-Shabaab forces were seemingly within days, or even hours, of taking control of Somalia's capital and realising their goal of an Islamist State.

In 2008, a combined force of African Union troops, making up the African Union Mission in Somalia (Amisom), arrived in Somalia charged with aiding Somalia’s Transitional Federal Government in its war against the Islamists. At first the foreign contingent enjoyed little success. Under-resourced it could do little more than hold key locations against the advance of the militants and became mired in a brutal, close-quarters battle with al-Shabaab in the narrow streets and alleyways of Mogadishu.

Image: Dec. 14, 2011 file photo, two Kenyan army soldiers shield themselves from the downdraft of a Kenyan air force helicopter as it flies away from their base near the seaside town of Bur Garbo, Somalia (AP Photo/Ben Curtis, File)

• 2011 - Present

As support financia and political for Amisom increased, in Africa and further afield, so too did military victories. A key moment in the conflict came in August 2011, when al-Shabaab left Mogadishu to the shock of even the most seasoned observers of the conflict. Their commanders labelled the departure a tactical retreat.

This has since proved to be the start of a new era in Somalia's recent history, leading to the formation of a new Somali government in 2012 – the country’s first with a legitimate claim to true authority over much of the nation in over 20 years.

Following the withdrawal from Mogadishu in 2011, al-Shabaab has gradually shifted towards more asymmetrical warfare. As Somalia as a nation has begun to find its feet for the first time in decades the nation remains teetering between the hope of a peaceful future and the instability that reigns as al-Shabaab frequently, and often devastatingly, attack key Somali government and Amisom targets, using tactics such as suicide bombers and surprise attacks. While Somalia, and Mogadishu in particular, is enjoying a level of safety and stability unlike anything it has seen in two decades, it remains a deeply dangerous and tense place as, despite territorial losses, al-Shabaab continues to be a potent threat.

Al-Shabaab is now estimated to have roughly 7-9,000 fighters, and continues to control much of southern and central Somalia, although has lost a series of key towns in the past few months to the advance of Amisom forces.

The aims of al-Shabaab

The aims of the group are debated by experts, while the group’s activites and tactics have evolved over time.

Initially the aims of Al-Shabaab were akin to those of the Taliban in Afghanistan in the late 1990s     – a war that was at once a separatist struggle and a religious one. The aim was for control and the founding of an Islamist State, with large scale engagements utilised as territorial gains and the holding of key strategic locations was key.

While declaring admiration for - and eventually a formal link with - al Qaeda, the group did not engage in a global jihadist struggle akin to that espoused by the group once led by Osama Bin Laden. And while foreign fighters did travel to fight in Somalia, for many years the group’s aims mostly focused on the localised conflict on the Horn of Africa rather than a global struggle.

Image: Dawn breaks over the still-smouldering Westgate Mall in Nairobi, Kenya, Thursday, Sept. 26, 2013, following an attack by al-Shabaab gunmen in which 67 people died (AP Photo/Ben Curtis)

Moving the war beyond Somalia

The first al-Shabaab attack beyond Somalia came in 2010, when militants killed 74 people watching a broadcast of a World Cup game in the Ugandan capital of Kampala. The attack was a reprisal for the involvement of Ugandan troops in Amisom.

“We are sending a message to every country who is willing to send troops to Somalia that they will face attacks on their territory,” a spokesman for al-Shabaab said following the attack.

Al-Shabaab’s loss of control and territory in Somalia in recent years has corresponded with an increase in activity beyond Somalia’s borders.

Since 2012 the group has carried out an estimated 17 attacks in Kenya, killing over 300 people. These attacks began in 2011, in response to Kenyan troops entering Somalia to join the battle against the militants.

Until Thursday’s attack, the massacre at the Westgate mall in Nairobi had been the group’s largest attack in Kenya, but in recent months there have been a spate of large scale killings, including the killing of bus passengers and quarry workers.

In the wake of the Westgate attack, then Al-Shabaab leader Ahmed Abdi Godane said via the group’s English language Twitter feed: “There is no way that you, the Kenyan public, could possibly endure a prolonged war in Somalia and you cannot also withstand a war of attrition inside your own country.

“So make your choice today and withdraw all your forces . . . [or] be prepared for an abundance of blood that will be spillled in your country, economic downfall and displacement.”

• Foreign fighters

There has long been a foreign contingent within al-Shabaab, while security agencies in Europe, the US and beyond remain concerned about the group's reach to the major Somali populations in London and the US state of Minnesota.

The man reported to be the mastermind of the Garissa attack, Mohamed Mohamud aka Dulyadeyn, is Kenyan, while witnesses to the attack reported that the attackers spoke Swahili, an official language of Kenya.

The first US born suicide bomber was an American-Somali named Shirwa Ahmed. Ahmed was a Minnesota born who killed 24 in an attack in Hargeisa, the capital of the unilaterally declared independent state of Somaliland, which is situated in what is still internationally recognised as northern Somalia.

The Somalia government has long made claims of al-Shabaab’s role in a wider global terror network, while calling for international military support in fighting the group.

Image: Members of Somalia's al- Shabaab militant group patrol on foot on the outskirts of Mogadishu, March, 5, 2012 file photo. (AP Photo, File)

Ties to other groups

Al-Shabaab have been formally linked to al-Qaeda since 2012. Informal links between the groups go back long before this however, with several of al-Shabaab’s leaders reported to have travelled to Afghanistan in the late 1990s to fight with the Taliban, and the many of the jihadists who fought in Afghanistan making the reverse journey to Somalia in the latter years of the last decade as they fought to establish an Islamic State in Africa.

However, al-Qaeda’s increasing struggle for relevance and influence in the face of the rise of the Islamic State (IS), a former ally but now rival to al-Qaeda, has led observers to wonder if the Somali group might pledge their allegiance to IS, as Boko Haram, the Nigeria based Islamist faction, has done. The deep ties between al-Shabaab and al-Qaeda however makes such an allegiance seem highly unlikely.

"First, al-Shabab has given allegiance to al-Qaida, the parent organization, and to throw that away will be difficult,” Abdiaziz Alas Artan, a Cairo-based scholar of militant organizations, told VOA.

"Secondly, al-Shabab feels it’s the older organization, the more senior one that has the priority, and if anyone has to move, [Islamic State] has to ... for them to downgrade and to join a group that, not just emerged just yesterday, but one that also disobeyed al-Qaida central, is going to be difficult.”