The issue of when to intervene into another country's affairs should be conducted under the guidance of international law and with the support of the majority of people living on the planet. Now that technology allows a very small minority to have a devastating impact on others, or even the majority, it has become all the more critical to work together and govern well. The international community must formulate a workable set of rules and procedures that we all can live and die by. If democracy and liberty for all is the best system, then the world government should abide by the same rules. It is time to improve the current, semi dysfunctional world legal system before large problems become more difficult to manage.
Nuclear proliferation is just one of the major challenges that a functioning world government could help prevent. Global climate change, world energy and food supply issues, population growth and communicable disease management are just a few challenges that we all have in common.
I personally believe that we have the technology and capacity to make vast inroads into these areas if we all work together. It will be exciting to watch and see how we all adapt together over the next 5, 10, and 20+ years. Things are changing so fast.