Today we live in a highly efficient global system. The degree of sophistication and specialisation of the economy is so high today that making a single product, for example an iPad, involves resources and facilities scattered literally all over the world. This minimizes the cost of the product, but if only a single facility fails, the product cannot be made. For example, look up the effects the recent flood in Thailand has had on hard drive prices, because a large fraction of the world's motors that move the disk plates have been produced in a few factories in Thailand. The shortage of a component that is as simple as an electric motor has caused a drastic rise in the price of hard drives.
Such a efficient and brittle system has little tolerance for disturbances, just as only a slight change in the environment can bring a highly specialised species to extinction. The inverse relationship between efficiency and resilience is the reason why the highly specialised cheetahs are doing badly compared with the more versatile lions in todays human-influenced environment, and why the highly versatile rats are thriving and are overcoming everything we're throwing at them.
All the world's major powers have rational governments today, and because they are parts of that brittle global system they will avoid disturbing it. That's why a war between major powers is not an option today. A minor skirmish, like an air raid against Iran's nuclear facilities, is still an option of course.
This all can change. The global system, as it is today, depends on lots of cheap oil to keep its far-flung components physicaly connected. Global crude oil production has been on a plateau since 2005. Despite an unprecedently long period of high prices (above 50$ a barrel), crude oil production is stagnating. What is growing are unconventional sources, like deep water oil and shale oil, which are more expensive to extract and need continuing high prices to remain profitable.
At a certain point, oil prices will become so high or production will become so low that global trade will become too expensive, and the global system will fragment into continental systems, which can be maintained more easily.
At this point, a war between major powers becomes a possibility, because the major powers no longer depend on each other. But still, only a possibility, because the fragmentation of the global system will be inevitably accompanied by an economic depression, and a rational leadership would avoid fighting a costly war in such conditions, and wait for technological progress to facilitate an economic rebound, and then try to re-establish a global system. Of course, progress is not guaranteed to materialise, or to materialise in the span of decades or centuries. Rapid technological progress has been more of an exception than rule in history.
And if progress does not occur, then we can expect continental economic systems to fragment into even smaller and simpler ones, whether they remain politicaly united or not.
Unfortunately, if a major power acquires an irrational leadership, then all bets are off.
