The world needs a powerful "police" country such as the USA. It is beneficial that the world has a military power that is willing to police the world and keep check on other countries military capabilities. Such a country has a stabilizing effect on the world. Such a powerful country can step in when serious threats to world peace and stability arise in the world, without the requirement of awaiting multinational consensus. Of course many of the USA's interventions are based around it needing to secure its own sources of oil, but aside from that many US interventions are necessary for other reasons.
It is not hypocritical for a country that has nuked a city to forcefully nullify foreign weapons of mass destruction. Nagasaki and Hiroshima were nuked by the United States of America, a terrible event that killed and injured hundreds of thousands more civilians than military targets. Some accuse the USA, therefore, of being hypocritical of now going to war with countries and accusing them of building weapons of mass destruction on account of these weapons being unacceptable.
But I believe the USA is right to do so. Even if the USA had the most terrible history, it would not make it wrong for it to make a correct stand now. For example, once an ex drug user came to our school and taught the class about the perils of heavy drug use. The fact that he was once an addict did not mean that his later talk was useless. In fact, it meant that he above others was qualified to make that talk.
The USA is very active when it comes to aid and support for poorer countries. Inequality will always exist and it helps to have a very powerful country that is rich enough to be able to step in and help in international emergencies and humanitarian crises. The EU is sometimes much more effective as it is closer to the continent but without US financial aid, many EU humanitarian missions would not be possible.
The USA has the technology and resources to develop new industry, technology, medical techniques and knowledge that would otherwise be discovered or created slower. Japan is the same.