I believe that the UN is very much relevant. Its predecessor, The League of Nations, was discontinued, but at least the UN has survived. I don't think we can expect miracles, but as long as there is a place to talk, there is reason to be hopeful.
In the US, the partial dismantling of the State Department has been very disturbing to me. (The State Department has been at least part of the topic for a couple of letters to the editor I have had published in the Washington Post.)
We all need to rely more on diplomacy. This is very important, globally naturally, but also at all levels.