regimes in Germany and Italy. Japanese totalitarianism had its roots in anti-colonial backlash as Japan sought to be the chief driver in Asia at the expense of the Western powers.

The end of WWII saw the return of a two-superpower world order as the United States and the Soviet Union sought to expand their influence through investment, military occupation, and influence. It was an uneasy peace as Truman and Stalin did not trust each other. The United States had the atomic bomb and the Soviet Union was not far from developing their own bomb.

WWI shook up the world order that had existed ever since the end of the Napoleonic War; it would take another world war to create a world that would be controlled by two major powers.

I would agree that breakdown of the multi-polar distribution of power between 1914-1945 was more or less inevitable. To understand what was going on, however, we need to head back to the nineteenth century. Through most of the nineteenth century, from the end of the Napoleonic Wars in 1815, Great Britain was the world's undisputed superpower. Britain had the largest, most powerful navy in the world. It was the undisputed ruler of the seas. It had a huge and lucrative empire, spread across the globe. It also was the first country to take-off into industrialism, giving it a huge head start in wealth creation. It is difficult to overstate how immensely wealthy and powerful the country was.

When one country is that powerful, it usually leads to peace, because no other country is suicidal enough to challenge its power. If a country does, it is quickly crushed, sending the message to other nations not to start trouble. Thus, through most of the nineteenth century, a "Pax Brittanica" prevailed. However, by the end of the century, Britain was showing signs of weakness. The Boer War was a mismanaged disaster, and other countries were rapidly catching up in industrial strength and establishing their own overseas empires. Most particularly, Germany unified. It gained territory in the Franco-Prussian war. It too industrialized and wanted to use its new wealth to challenge Britain's control of the seas.

As Germany strengthened and Britain weakened, struggling to hang on to its top-dog position, a clash was almost inevitable. It came in the form of World War I. That war, however, failed to solve any fundamental problems. Power became multi-polar, Britain continued to wane, and Germany rearmed under Hitler. The United States, which everyone understood was poised to be the new super power, remained in the shadows, immersed in isolationism until World War II. This unstable situation, in which it was unclear who was going to win the global power stakes in Europe, led to the war that finally rearranged the global European order.

This is a very hard question to answer with certainty because we, living today, know how things turned out.  We naturally have a bias to think that the way things turned out was inevitable or was, at the very least, highly likely.  Even though I am aware of this potential bias, I still think that the breakdown of the multipolar international system was highly likely. This is true because of the fact that there were countries that wanted to upset the status quo and gain more power than the system allowed them.

A multipolar, balance of power type of international system can be very stable.  Such a system works because the powers in the system work to make sure that no one country gains too much power.  But this becomes more problematic if countries that are not major powers believe that they should be major powers.  If these countries can gain allies (from inside or outside the current system) they can disrupt the system as they try to gain power.  This is what happened in the time period that you ask about in this question.

During the time between WWI and WWII, three countries that were not really powerful wanted to gain power.  These were Germany, Japan, and Italy.  Germany had been a major power before WWI, but it lost its power in that war.  It wanted to regain power.  Japan and Italy had not been accorded major status before the war and they were not seen as major powers after the war.  They did not like this situation and wanted to increase their standing in the world.  These three countries were able to join forces and disrupt the status quo, causing the international system to break down.   This is, in my view, a highly likely scenario when there are outside powers that aspire to gain more power in the international system.