The idea that the United Nations could work as a world government is pretty laughable. No matter how closely intertwined the world's nations become, there will always be some level of sovereignty. No nations would willingly turn over its decision-making ability to a worldwide governing body. And smaller is better in government. Even Canada, a small industrialized nation, is facing a possible breakup.
Small towns have problems with effective government and reaching consensus among the governed.
Just imagine trying to govern 5 billion people. Then there is the peacekeeping mission, one which the United Nations has attempted to perform with varying success since the 1950s. When the United Nations attempts to bring peace to an area where there is still fighting and no one wants to cool off, the United Nations looks like a squad of clowns. U.N. command, with its various restrictions on when and how the troops on the ground can engage local combatants, is unpopular with every nation which contributes forces to a peacekeeping mission. A U.N. army, suggested by some to alleviate the command problem, is also unworkable.