Your premise is false. The UN has intervened in numerous conflicts. There are a number of current peacekeeping missions in the world. The UN has even authorised military action to bring peace and order.
The reason that the UN can’t pass a resolution against Russia is the same reason it didn’t against the US when it waged wars of aggression. Russia has the power of Veto in the UN Security Council where resolutions are voted upon.
The UN is not a group that maintains peace. It’s more akin to a group-chat on whatsapp. You get to communicate with other nations, and settle disputes via talks rather than via military conflict – but military option is and always will be there.
until the day a country (or indeed all countries) decides to give up military sovereignty, the UN will never be able to actually be capable of keeping peace and security.
It’s a lot like having a deadbolt on your door. It’s not absolutely going to prevent anyone from ever breaking in. But without a deadbolt, the chances of someone breaking in are increased. Part of that is because just having a deadbolt visible on the door acts as a deterrent. And part of that is because it makes actually breaking in even more difficult, which makes it more likely likely the culprits will get caught and punished.
The existence of the UN, and its activities, serves to deter some nations from doing some bad things. And, when nations do bad things, the UN can increase the chances that the evil-doers will eventually face some kind of justice. But like a deadbolt, the UN cannot prevent every nation from ever doing bad things.
The UN does a lot of great work and is preventing wars either by providing a diplomatic channel, help provide consistent rules, being a neutral part of a conflict or in rare cases intervene directly with the support of all their member states. But one issue with the UN is that it is based on the principle that all nations have autonomy over themselves. So there are mechanisms to prevent unwanted foreign influences. The UN can therefore not pass a resolution without all member states agreeing, even the nations who are part of the conflict. It is governance by agreement taken to the extreme.
That does not mean that the UN is standing passively in the Ukrainian invasion. Putin justifies his invasion by claims of drugged nazis in charge and population massacres. The UN have a system to handle such genocides so the UN starts the work which could result in UN troops being sent to Ukraine instead of the Russian commanded troops. But Putin is one of the few who vote no on such resolutions going against his own rhetoric. Putin can no longer seriously claim that the invasion is to keep the peace and prevent genocide because he actively prevents the UN from helping him in these efforts. That makes it hard for others who might get out of picking a side by saying the invasion is justified.
They’re not “always” afraid. It’s massively dependent on who is doing the invading.
Small, non-nuclear nation with a limited army and no protection treaties invades someone? Probably likely that the UNSC passes a resolution authorising troop deployment to end the conflict.
When a nuclear country with significant armed forces invades someone it is entirely different as escalating the conflict could be magnitudes worse in terms of death and destruction.
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