Eventually, and it may take another couple hundred years, world peace will happen. When individual countries have equal rights, stable economies, equal minimum labor standards and viable governments, a worldwide form of government will be the end result. Any armed forces will have to act as peacekeepers instead of invaders. Civil rights and liberties must be upheld. It will happen eventually and it will be a good thing. Honoring people's differences while maintaining civil liberties in a worldwide government and non-poverty economy is as close to a Utopia as this planet can get.
No, it is not justified to have worldwide hegemony, because a system like that would always be misused. The richer nations would end up paying for the entire thing, and the poorer nations would continue to demand more and more services. This would become an agenda for social control, rather than simply for centralized defense.