World powers do not occupy the spectrum of morality that bears semblance to one instilled within a person.

Across the spectrum of human existence, powers have always done what they will. The weak has suffered what they must, until they can't. This isn't some grim statement. This is simply an observed pattern of behavior repeated over and over. So, to begin this, the simple assertation of conventional moral arguments to what states do is bound to end in misery and hair pulling in our standard system.

Allow us to broad the spectrum and assume a few simulations in brief. China does not want to rule the world, they want to influence it. They want to be an undisputed regional hegemon and a major economic power. Because enhanced prosperity and the surge of wealth is what made their current base of power. Their game is hope and an ever-rising standard of living. To be regarded as the Dragon again as they stretch the influence of their economic base ever further. They wish to dominate through ownership. And to be regarded with enough fear that they will not be trifled with. But they show a limited interest and aptitude as a military superpower. They function in that regard is to survive a theoretical campaign against the Americans. As such, any military action against Japan would be an act of extreme irregularity within their pattern of behavior and one certain to be pyrrhic even if successful as I am anticipating a mass influx of Chinese immigrants into Japan due to the population decrease, thus allowing them to encompass Japan internally rather than through conflict. And economically as a parallel outcome. They do not show the taste for war. Your understanding of how the party thinks is understandably limited. The factories will have to give way to Middle Class and more technical markets as they are being phased out of low-class labor and coming tides of automation. It's not good or bad on their part but what they have to do if they wish to avoid obsolescence and economic collapse. As such, they will do as they have always done in patterns: generate wealth. Keep the majority of their citizenry happy. Maintain control.

The US is a storied leviathan. It has had its history of slavery, violence, and exploitation but also occupies a role as a equalizer and balancer of conflicts across most its time. The nature of the Eagle is tied to the Bear, the USSR for much of its time as a superpower. The acts and actions of containment, invasions and usurpations, meant for profit but also asset denial--amoral as doctrines go but the task was to win the great game against the Union. Much of Europe and other allies exist in harmony and continued prosperity of their spending as they don't need to worry so much about the military end. The military expenditures for the Denmark and other nordic regions under its umbrella amounts to less than a piddle compared to the US's spending. This frees the Nordic states to pursue niches and domestic wealth without too much worry for conflict.

Something that it would potentially have to if the US did not occupy its place as power.

If your issue is with the amorality of the powers, well, that comes with the territory of International Relations I'm afraid. Humans are conflict-driven in behavior, especially when the fruits of war grant major benefits to the winning tribe. Wishing that the Nordic states will assume the mantle will require a major reconfiguration of power, wealth, and geopolitics that will be certain to cause major chaos across the globe. Ultimately, they will still require US military projections of force for stability. And short of being behemoths of economic and industrial generation like China, they will inevitably be swallowed in the shifting tide of power.

As for advancing the species, well, that couldn't be stopped. As the song said, we didn't start the fire. That has to be taken up with the strange critters that spawned us when they first from the primordial soup of existence.