Regarding your comment "would much rather prefer fixing the UN, than joining the NATO". I don't understand how you can compare them. I see NATO as a military alliance and the UN as communication platform for countries (writing and voting on resolutions). I don't see where one can replace the other.
Given that, I like the tone of OP's point - why can't the UN members (which is all countries right?) come together and just establish a list of sovereign countries and their borders and then humanity can be done with invasions? I realize there would be some tough conversations, maybe intractable regarding Taiwan, possibly other areas as well. Start the conversation though. Move humanity from focusing on military pacts to economic/aid pacts and focus on human rights.
The UN has already done this to the degree agreement is possible. Without the military power to enforce those boundaries, the agreements are useless.
That assumes a lot of trust between all nations, their leaders and potential future leaders. A military alliance is just the manifestation of stronger bond between particular nations. With humans being the way they currently are I don't see this being realistic without larger societal changes around the world.