The UN has conducted many successful peace keeping interventions. Sierra Leone was but one. The problem Wars are big news, no wars aren't.
The UN is very much a sum of its parts. It has 193 members and a Security Council of 15, with 5 permanent members and 10 members voted on and off. It only takes one vote in the Security Council, to defeat a motion. Currently Russia is causing the problem in getting a truce in Syria.
The remarkable thing is not those cases where the UN has been unable to intervene, but the large number of occasions where it has got all, or most countries to agree on peace keeping measures and then got individual countries to provide both men and money to do the job.
The British army provided all the troops and logistics for the Sierra Leone intervention, which was so resounding a success.
here is a list of UN peace keeping missions
en.wikipedia.org/wiki/Timeline_of_United_Nations_peacekeeping_missions
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