Nothing is forever. Times change, situations change, and government policies change. Nobody likes change, particularly in regards to international relations. The trouble is, governments are run by people, and as the people running the government change, so can policy. Monarchies provide stability for the life of the monarch. Dictatorships provide stability for the life of the dictator, which is usually not as long as that of a monarch, dictator life expectancies being what they are. Policies of a Democracy have the possibility of being even more ephemeral, since the ruling party, and even the current ruler of the ruling party, can change every few years.
It has been said that one of the admirable things about America is the stability of its international policies. Yes, it has had major policy changes — NEWT, PRC — but those were understandable changes in response to observable changes in the international situation. Once a new policy comes in, it tends to remain relatively stable across administrations and parties. Unfortunately, that no longer holds.
From his earliest campaigns, Trump has been opposed to US participation in multinational organizations, particularly NATO. He has displayed a total inability to understand the reasoning behind such agreements, grounded in his inability to understand the concept of a military in general, and a dislike of any agreements that are not transactional. The latest reports of his earlier statements, that as President he would let Russia do whatever the hell they want, are emblematic of his thought processes. There are two problems with this, one immediate and tactical, and the other long term and strategic.
Most immediately, his remarks signal to the aggressor nations of the world that they now have a free hand to take whatever actions they want, presumably as long as they do not directly attack US assets. So, Russia can continue to attack Ukraine, and move from there to Latvia or Poland. China can attack Taiwan. North Korea can attack the South. Even aggressor use of tactical nuclear weapons are not clearly off the table.
More concerning, and more long term, is the damage to the image of the US as a strong and above all firm ally. It’s no longer just about Trump. What he has made clear is that any US President can overturn the entire international system if he changes his mind, or even in a fit of pique. Now, this has always been true, as true as it is of democracies in general. But for eighty years the international interests of the US have militated against such actions. US Presidents have not taken such far-reaching changes, because they are US Presidents, and realize the long term impacts on US interests. Until now.
Trump is the least qualified man ever to become President. He has no concept of the functions of the office, and is not interested in learning. He does not understand what he has done, even though he is out of office and is not likely to get back in. But the damage is tremendous.
Consider. You are the leader of a smallish country, dependent on international trade and threatened by an aggressive and expansion-seeking Russia or equivalent on or near your border. You can no longer trust the US to come to your aid in the event of an invasion, and you can’t even trust America to keep to the terms of any economic trade agreements. What do you do? You cast about for neighboring countries in the same economic boat and you build a local and regional trade network that is specifically designed to function without US involvement. If the US comes seeking some sort of a trade agreement, you will be much more hard-nosed about it — if the US wants a transactional agreement, then you will give away absolutely as little as possible. As for Russia. If you cannot depend on US support, then you have to be prepared to deter Russian aggression on your own, and the only way to do that reliably is with your own nuclear weapons. Suddenly, Poland, Germany, Turkey, Taiwan, South Korea, and Japan all need to start thinking about the nuclear option in a non-US world.
The problem is, this is the world we now live in. Trust is a fragile thing, and Trump has destroyed it. Not as completely as he would if he became President again, but enough that all the countries that sheltered within the US system, and took directions from the US will now start edging for the exits. Nuclear proliferation will expand from Russia/China/North Korean clients to other countries around the world. The US, and the presumably-non-US United Nations might object, but the incontrovertible reply will always be, we can’t rely on you, so we have to rely on ourselves. That’s the irreparable damage that Donald Trump has done.