What threat does the establishment of an Islamic State by ISIS pose in the region and the world?
As far as i know they took control of a large portion of land in Iraq and Syria, however, they are considered as a terrorist organization by much of the world. They also have powerful armory (tanks, machine guns) and money resources according to Wikipedia.
Clearly ISIS is a threat to the population in Iraq and Syria. But it doesn't pose a threat to the World. In the worst case scenario the World may suffer some more 9/11 style terror attacks (which is unlikely as any such plots will likely be stopped well before they can be executed). But note that while 9/11 was perceived to be a huge game changing event and led to the US to embark on the War on Terror, it actually didn't even make a dent in the US' military strength.
If anything, the political resolve after 9/11 made the US de-facto a stronger force. Also, it led to improved relations between the US and Russia (the US objected less to the Russian military intervention in Chechnya). Similarly, I think that if ISIS were left alone and they were to establish a califate in Iraq and Syria, the World would see that as a big threat which would lead countries to put their differences aside, which would benefit most of the World.
Not the ONLY threat by any means, but a state that is a safe heaven and officially supporting terrorism (especially Islamist kind) has proven to be a bad idea - With the resources and support of a state the terrorist groups can carry out much more impactful attacks (aka 9/11, once Al-Qaeda found safe heaven in Taliban controlled Afghanistan, but that is by no means the only example).
A second threat is that this upsets the balance of power between Shia and Sunnis. If Sunnis manage to rub out Al-Assad in Syria, and take over most of Iraq, especially its oil production, you would have a super powerful Sunni block from Syria to Iraq to parts of Palestine to Saudi Arabia to Qatar (I'm sure I missed a state or two). Add on the likelihood that they can rub out Jordanian kings without too much threat, and the chances that Moslem Brotherhood can still take over Egypt again if Obama keeps playing his strange anti ISIS games, and Sunni power block becomes unbalanced largely and under the control/influence of the same Wahhabi branch of Islam that gave the world the pleasures of Al-Qaeda.
In addition, the final goal of IS is worldwide domination of extremely hard-line Islam, by force of arms. Whether they have a realistic chance of attaining that goal is debatable, but if you look at the geopolitical successes of early Caliphates and the reach of Muslim dominion at its peak, it's not a completely idle threat.
