I hate how “anti-war” has been hijacked by these people to mean, let imperialist countries invade whoever they want with no consequences. (in the case of tankies, any imperialist country that isn’t in NATO).
Similarly if most countries have a mutual defence pact, no one country will be able to invade another without being at war with literally the whole world.
I'm afraid that mutual defense isn't as iron clad as you think. If Article 5 of NATO ever gets triggered you'll get a masterclass on weaseling out of obligations. It's ironic because Ukraine may already be receiving the kind of support a full NATO member is entitled to.
But imagine the implications if a country did not commit to it (bar an obvious one like Hungary or Turkey). They’ll likely get sanctioned, probably will have trouble entering any useful alliances for the next decade or so because no one trusts them anymore.
The trick is that (Eastern) Europe is filled with NATO troops and material from all countries. You would need an extremely nasty retreat of these troops if you do not support (say) Poland. Also at the moment (officially) the US has stationed nuclear weapons in 6 European countries, and there are very likely more also in the form of submarines that are not known to the public. Retreating means leaving those weapons in Russian hands. Then again, maybe Trump does not care about that.
Well ideally if you don't want to be a part of an agreement, you just tell the people you made the agreement with that you are no longer a part of it.
If you do so amicably, parties may be fine with it it may not have bad effects. If you wait till one of those counties is being invaded and back out, it likely would not end amicably, and with them having to switch over to a wartime economy, they may cut all trade moving forward with the member who screwed them over. Could cost the U.S. trillions in trade annually.
Aka it would be more profitable to support your allies, or get out of the agreement early, but that doesn't guarantee counties don't say... Why should we trade with someone who would hang us out to dry? And it hurts our economy anyway.
Ireland has peacekeepers between Israel and Lebanon right now. They wouldn't be there if "create peace" was one of the missions.
Peace must exist, however briefly, and then the peacekeepers place themselves in harm's way to keep the peace.
Extend the mission to militarily "create peace" and suddenly you are just NATO/USA. How can either side trust a peace that was enforced upon them and not call it a defeat, whose borders are disputed for eternity? Every nationalist who wants to stir sentiment can just say "look what the British/Americans/UN imposed on us". Outside forces drawing borders is pretty much the cause of 90% of warfare, civil and otherwise, for the last 80 years.
A Canadian was also the principal author of the UN declaration on human rights, and another was the reason NATO is a political as well as military alliance. We just keep winning!