I can't help myself not to find Buddhism utopic.
If one assumes that it's purpose is to make the earth a place with no violence, or to make peaceful beings not die from life-threatening attacks, it is utopic. But that assumption is incorrect.
What is the long term plan of the Buddhism?
Buddhism is not an abstract view of society. It does not put forward any grand vision of what an ideal society should be.
But within a society (a real one), it offers useful principles to guide human affairs. The utility of these are always within the theme of decreasing suffering of people.
How would Buddhist monks organize countries, world?
A monastic person is one who put aside most wordly matters, which include governing. Having them organizing countries, the world, would be to put back on their plate what they specifically put aside.
For a historical understanding of how buddhism have being put to use on government affairs, I suggest as a starting point of research the emperor Ashoka.
Imagine you got 95% Buddhist countries, but ISIS pops up, how would Buddhist countries react, given that non-violence is preached?
This is a very speculative question, which not only is something that buddhists are taught to avoid, but the policy of this community also does not promote developing such discussions. However, if this question's subject is reduced to an individual instead of countries, the dilemma might be preserved and maybe a satisfying answer can be given. So, to the reformulated question:
how a buddhist person reacts to a life-threatening situation, given that non-violence is preached?
...while it's hard to say how buddhists reacts (any attempt in my part would be factually wrong), the Buddha taught abandoning ill-will. Note I'm not saying buddhists react with non-violence. I'm saying that the Buddha drew a very strong line on this matter1 (and its up to each one to decide to live by it):
"Monks, even if bandits were to savagely sever you, limb by limb, with a double-handled saw, even then, whoever of you harbors ill will at heart would not be upholding my Teaching." (MN 21).
To conclude, Buddhism is not something that needs the agreement of everyone (or from a large group of people) to operate satisfactorily. It's not about society. It's a road for an individual to decide if that is the kind of person he or she wants to be.
1 this is not without it's nuances and polemics, say, when it comes to the use of violence in self-defense or helping others by means of harming. Without going into detail, what seems generally accepted is if the state of mind that manifested violence came from greed, hatred or ignorance, it is blameful.