Storm in a Vacuum
The importance of stability, legitimacy and capacity for a government to keep the domestic peace has been discussed at length here. Equally important is the affect these have in the international sphere. Every government always tries to increase its dominion for its own benefit, be it the pursuit of resources, land or people. A government's ability to expand or protect its dominion is dependent on its own stability, legitimacy and capacity, but it is also dependent on the stability, legitimacy and capacity of its foes and neighbors.
In European history, great powers often aligned with other great powers to prevent one great power from becoming too strong. The most notable example of this was during the reign of Louis XIV of France, when England, Spain, Austria, the Holy Roman Empire, Savoy and the Netherlands all banded together in a Grand Alliance to prevent the Sun King from eclipsing them with his military might. Maintaining a balance of power on the continent was of particular importance to England, which based its entire geopolitical outlook on keeping continental Europe fractured. This pragmatism was not confined to the English - the Thirty Years War was ostentatiously about Catholicism and Protestantism vying for dominance, but Catholic powers and Protestant powers often allied to defeat a common rival that adhered to the same creed.
Technological progress gradually gave powers faster transportation and communication, while also giving them a martial advantage over people that did not share in this progress. The spread of European global empires would continue until World War I, when the cumulative wealth of hundreds of years of production and plunder was thrown away to fight a futile war that nobody won. The League of Nations that was supposed to keep the peace after this cataclysm was unable to enforce its will, which enabled Japanese, Italian and German aggression that would culminate in World War II.
World War II ended with the advent of superpowers. Superpowers are governments that have the ability to project their military power worldwide, and in the subsequent Cold War the Soviet Union and United States overthrew governments and fought proxy wars, but did not come into direct conflict. This was in large part due to technology - the development of thermonuclear weapons made extinction the end result of another world war. This has resulted in an unprecedented era of relative peace that has nothing like it in recorded history. After the fall of the Soviet Union and end of the Cold War, this era continued with the Pax Americana - the era of unquestioned American dominance over world affairs.
This era seems to be coming to an end. President Trump has been making hostile overtures toward many linchpins of the American World Order. After ending military exercises with South Korea in the wake of the Singapore meeting with Kim Jung Un (where America got absolutely no concessions from Kim), calling the European Union a foe of the United States (of which most of its members are part of NATO), America's withdrawal from the United Nations Human Rights Council, and favoring Putin's claim of innocence for election tampering over the findings of 16 different American intelligence agencies and the judgment of the Senate Intelligence Committee, the United States is visibly in retreat from its commanding position in the world. The American World Order that has been the framework for the world is falling apart, and it is becoming more apparent that the phrase "America First" seems to mean America stepping off the pedestal.
An America not willing or able to protect its interests naturally invites others to take advantage of the situation. The two biggest winners in this retreat are China and Russia - China has been heavily investing in Africa and Central Asia, and while middle America might think of these areas as "shitholes" with no redeeming value, they have titanic importance in global trade that is realized by Chinese leadership. An alliance of South Korea, Japan, Taiwan and Vietnam may counter the rise of China, but the fact is that without the United States to back them up, the cards are heavily staked in China's favor. China is well on its way of crafting a Chinese World Order on the ashes of the American World Order.
Russia, for its part, will become more assertive in its territorial acquisitions. After stealing South Ossetia from Georgia and the Crimea and Donbass from Ukraine, Russia has been menacing the Baltic nations of Estonia, Latvia and Lithuania, all of which have a sizeable Russian minority. Areas populated by Russian speakers were considered a casus belli for Russia's earlier invasions; what has stopped Putin from doing the same in the Baltic states is the fact that all three of them are inside NATO. Article 5 of NATO stipulates that when a member of NATO is attacked, it is as if all nations in NATO are attacked, thus obligating a united defense. This has been utilized only once in its history - after the September 11th attacks. Russia would be able to steamroll all of the Baltics in days; it can not do the same with the rest of Europe. This is why Trump kicking the tires on NATO is a big deal - if the United States pulls out of NATO, NATO loses its strongest deterrent against Russian incursion. The dissolution of NATO and the European Union would be the greatest gift Putin could ask for.
One can not credibly say anything with certainty about the future, but with no domineering power in the world, regional powers will aspire to become superpowers. And as regional powers clash in this aspiration, the apocalypse that was suspended in the Cold War risks a return to relevance. If America does withdraw back into a shell, the world will likely have China be the most important country in the world, Russia the most aggressive country in the world, and Europe either shattered into pro- and anti-Russian alliances or united vigorously against Russia. This is not a formula that encourages peace. The biggest reason the League of Nations failed in containing the belligerents that would cause World War II was because the United States did not join it after World War I. We have seen what happens when America retreats. It must not happen again.