We have learnt from our past, the world is a more smarter and more cooperative then in the past. The united nation haven't prepared for it becuase it won't happen again, a war may start, and many wars have started but none of them have drawed mot of the world into them.
The united nations has made it clear there that they have allowed "veto" powers to the P5 countries. These include: the United states, United Kingdom, Russian Federation, France, and China. If a conflict arrises between any of those permanent members, that country can veto any resolution that is stated. So, no, I don't believe that the united nations is prepared for a world war. An example of today's events include the situations with Ukraine and Russia. Russia is vetoing any resolution that doesn't coincide with their ideals.