How would Buddhist monks organize countries and the world?

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.

We do not have to wonder about the answer to the question of "How would Buddhist monks organize countries, world?" We have plenty of historical examples of just this. There is no need to speculate. In Tibet for example.

"Until 1959... around 98% of the population was enslaved in serfdom. Drepung monastery, on the outskirts of Lhasa, was one of the world's largest landowners with 185 manors, 25,000 serfs, 300 pastures, and 16,000 herdsmen. High-ranking lamas and secular landowners imposed crippling taxes, forced boys into monastic slavery and pilfered most of the country's wealth – torturing disobedient serfs by gouging out their eyes or severing their hamstrings." The Guardian. 11 Feb 2009.

Other Buddhist countries are no better. Buddhist monks are currently leading the ethnic cleansing in Burma for example - openly and actively encouraging the murder of Muslims. Buddhist monks supported the government of Sri Lanka in their genocidal war against the Tamils (who had only started fighting after decades of persecution and injustice). Even the happy Bhutanese are persecuting Nepalese refugees - not forgetting that in nominally Buddhist Nepal there was a decades long civil war. Japanese monks encouraged the imperial ambitions of the Japanese government in the early 20th Century, supporting the invasion of China and what they call The Pacific War (aka World War II). Pacifism was finally imposed on Japan not by Buddhist monks, who had been there for 14 centuries supporting one militaristic government after another, but by American generals and nuclear weapons (as sick as that was).

Of the modern countries that are notionally Buddhist or have a majority of Buddhists, we have Sri Lanka, Burma, Thailand, Vietnam, Cambodia (Jesus, Cambodia!), Korea (Nth and Sth), China, and Japan. A brief look at the history of these countries is not encouraging when it comes to the political influence of centuries of Buddhism. The characteristic government of a Buddhist country in practice would on average be a repressive totalitarian state. The worst of them are also militaristic and imperialistic.

Just going on the real politik of Buddhist states that we know about, in a world that was 95% Buddhists and ISIS popped up we would almost certainly just systematically slaughter them. But since we would almost certainly have suppressed their religion and ethnically cleansed their region to begin with, they probably wouldn't have popped up.

No one who has even a beginners knowledge of the facts of history in Buddhist countries could sustain a Utopic illusion. But most of us prefer to remain ignorant and keep our illusions. The best chance for world peace is the real tolerance that comes with secularism! Compare also Stephen Pinker's book The Angels of Our Better Nature, which not uncontroversially, tries to prove that violence has decreased with the spread of rationalist secularism.