The UN as in international body has failed to resolve conflicts globally.?

You could argue that it hasn't and provide conflicts such as Vietnam, the Falklands, Iraq-Iran, Iraq, Rwandan genocide, Pol Pot, Sri Lanka, Yugoslavia etc. This would easily support the argument that it has been ineffectual.

However, to a large degree these are civil ethnic wars. There has been little inter-state warfare on the scale of wwii since the UN's inception. By and large the major powers have not been involved in open conflict; hence the phrase Pax Americana and The Long Peace. Both phrases refer to period of peace between major powers. You should look at the UNs role in this peace. Importantly, to get a top grade, you should look at what causes conflict and threatens security; migration, poverty, changing social and poltical environments, economic stagnation, have major influence in causing war and threatening security. how has the UN mitigated these threats? You can look at the peace keeping exercises in the Balkans and Iraq for support.

Instead of asking random people for opinions, I think the point here is for you do research, and then look at the evidence and decide what the evidence shows.

A thesis can't be supported by anonymous opinions. It has to be supported by actual evidence.