One of the world's most dangerous al Qaeda-linked groups is having an alarming resurgence

In September of 2012, Somali and African Union troops moved against Kismayo, the last major city in the country still under the control of al Shabaab, Somalia's al Qaeda franchise. As the offensive approached, Shabaab beat a tactical retreat into the country's hinterlands, ending a period in which the jihadist group controlled substantial territory and represented the country's most powerful political and social force.

It's been nearly three-and-a-half years since the African Union force hastened the end of al Shabaab 's state-building project. But the group made a canny strategic decision, contracting into defensible territory, consolidating its safe haven, and evolving into a deadly and resilient insurgency.

Shabaab lost much of its land, but helped ensure its ability to kill on a massive scale for years to come.

Shabaab's success calls into question whether the ISIS threat will be contained even if the group loses its strongholds in Mosul, in Iraq, and Raqqa, in Syria. Like ISIS, al Shabaab was a significant territorial power squaring off against a multinational military force purpose-built to dismantle it. And like ISIS, Al Shabaab has an external attack infrastructure, proven battlefield capabilities, and trans-national influence and reach.

Over the past month, al Shabaab has demonstrated its endurance, even despite territorial losses, infighting, and the death of key leadership. Three incidents show just how threatening the group still is. Together, they represent an ominous precedent for the future of organizations like ISIS.

On January 15th, al Shabaab overran a Kenyan military base in el-Ade, in southwestern Somalia, killing as many as 60 soldiers. The Kenyan military pulled out of el-Ade 11 days later, effectively ceding the territory to the jihadists.

The attack exposed alarming weaknesses within the African Union military mission, which has succeeded in removing al Shabaab from Mogadishu and restoring the country's internationally recognized government.

Shabaab delivered an apparent battlefield defeat to a western-backed conventional military, entering the Kenyan Defense Forces base through a frontal assault and then killing scores of soldiers during the ensuring firefight.

The aftermath of the assault is equally significant. Kenya ceded territory that could be considered vital to the country's national security. El-Ade isn't far from the Kenyan border, and the country has been the site of deadly al Shabaab atrocities in recent years, including the September 2013 Westgate Mall attack in Nairobi, in which 67 people were killed; and the April 2015 massacre of 147 students at Garissa University, in eastern Kenya.

That Kenya pulled out of a place like el-Ade calls into question the state of the African Union mission - and suggests that al Shabaab is still formidable on the battlefield.

Just a few days later, on January 22nd, Shabaab gunmen stormed a beachside restaurant in Mogadishu, killing as many as 20 people. Unlike the el-Ade attack, the Lido Beach assault was focused on a soft civilian target, and one with particular resonance for citizens of the Somali capital: the beach is one of the city's few remaining leisure spots. Al Shabaab is also infamously hostile to most forms of beach recreation, bombing beachside restaurants and banning soccer when it ruled over the city.

Mogadishu has been a frequent target of al Shabaab attacks. But it's also seen its economy expand since Shabaab lost control of the city in 2011, thanks in part to the 2012 return of the Somali government and Somali diaspora investment.

The Lido Beach attack showed that al Shabaab can kill still dozens of people in the city's equivalent of Central Park.

Finally, on Tuesday, an apparent bomb blew a hole in the side of a Somali passenger jet traveling between Mogadishu and Djibouti, killing one passenger and forcing the plane to make an emergency landing in the Somali capital. On Wednesday, Reuters reported that investigators believe al Shabaab is responsible for the blast.

Sneaking a bomb aboard a commercial airliner is a difficult feat just about anywhere in the world - and the Mogadishu airport zone is a heavily guarded area. The airport is considered one of the few places in the city that foreign dignitaries can safely visit. It also encompasses offices and residential areas that are physically walled off from the surrounding city, a veritable "green zone" whose tenants include Bancroft, a US-owned security contractor.

Al Shabaab didn't just bomb a commercial plane. It might also have managed to smuggle an explosive device inside the most heavily guarded location in the entire country.

If Shabaab was responsible for the suspected bomb attack, it would mark the first instance of the group detonating a bomb onboard a passenger plane. Like the other two attacks, it would be a gruesome sign of the group's resilience - and of its increasing danger years after the height of its territorial power.

Reuters contributed to this report.