experience
Well, the palestinian terrorists of the 70's and 80's (like Abu Nidal) or the ETA in Spain or the IRA in Ireland never were an argument against the United Nations.
The UN's mandate is about preventing a country from invading another (like Iraq invading Koweit).
But when the enemy is not a nation but an international network of terrorists, this requires other international measures.
Probably the UN could become competent in dealing with this, but it would require an intitiative from its members, at least the Security Council.
Raoul