(CNN)Five of CNN's correspondents gathered recently to discuss how the world changed in 2015 and what may come in the year ahead. Nick Paton Walsh, Nima Elbagir, Ivan Watson, Clarissa Ward and Arwa Damon sat down with videographer Claudia Morales and talked about the stories, issues and people they had covered. See their conversations in a series of videos and catch up on the key stories of our time. Here, ISIS. Also in the series: The refugee crisis; Russia
ISIS: What happened in 2015 and what's ahead
They are known by those who oppose them as ISIS or ISIL (Daesh in Arabic), but they call themselves the Islamic State, a name that signifies their self-declared ideological claim over the Muslim world and their bloody ambition to force their perverted interpretation of the faith on all 1.6 billion of its followers.
Born in al Qaeda's war in Iraq, bred by the Syrian conflict, bastardized by Al Qaeda, which disowned the group, and globalized by its shadowy leader Abu Bakr al-Baghdadi, the terror group now controls a territory roughly the size of Connecticut, stretching from northern Syria into central Iraq. Known as a "death cult" for their nihilistic vision that preaches the end of the world is near, ISIS jihadis believe they will soon fight the battle of Armageddon -- and win.
In 2015, this self-proclaimed caliphate brought new horrors to targets from West Africa to the heart of Europe, and it promises to be one of the greatest challenges of 2016.
This year ISIS institutionalized life under terror, held on to most of its territory, established a government bureaucracy that includes everything from birth certificates to a gold-backed currency, and grew its already prolific propaganda strategy. These jihadis that U.S. President Barack Obama once called a "JV squad" wavered little under airstrikes by the U.S.-led coalition.
In fact, ISIS expanded by adopting or allying with other regional terror groups, including Boko Haram and Taliban offshoots in Afghanistan. Extremists seemed to compete among themselves for the most brutal attacks carried out or inspired by ISIS: the bombing of a Shiite mosque in Yemen that killed 137; the downing of a Russian passenger plane over Egypt; attacks on tourists in Tunisia.
2015 is the year ISIS showed it could be anywhere -- or, if it didn't have presence, there is always the "lone wolf" model. These nearly undetectable types of attackers, such as the shooters in San Bernardino, California, have proven problematic for intelligence officials.
Jihadi recruitment now happens in homes -- what analysts call "bedroom jihad" -- or online, away from the communities that could raise the alarm, making it even more difficult to stem the bloodletting. The greatest shock came in November, when ISIS carried out a series of coordinated attacks on Paris that left 130 people dead and much of the world horrified.
The terror attack on France may prove to be a turning point as next year's fight against ISIS unfolds. The Western world's own values are at risk in this war -- from the humanitarian obligation to refugees seeking asylum, to the future of open borders among EU nations. The group is sure to face renewed bombardment from the West, but destroying ISIS' infrastructure will not be enough to crush its ideology.
The recent Russian intervention on behalf of Syrian President Bashar al-Assad may prove to be another obstacle; many experts say the Kremlin's offensive is sure to weaken Syria's rebels, one of the few challenges to ISIS on the ground.
But it's not all about guns.
As the United States elects a new president in 2016, diplomacy will come into play if the gap is to be bridged between the proxy countries involved in the conflict, such as Russia, Iran, Saudi Arabia, and others. Political leaders will have to work to convince key actors on the ground, particularly the marginalized Sunni population in Iraq, to expel ISIS from their towns and cities. And the world has yet to agree on a plan to end the war in Syria, where a vacuum has fueled the depravity of the organization.
The world is unlikely to rid itself of ISIS in 2016 -- the battle is simply too great. But it can begin to starve out one of the most wicked groups of the 21st century.