Joining together there is cooperation,peace, economic development,prosperity, and less conflict. Look at example of EU. The cooperation among the EU has risen the standard of living of all its citizens and bought peace. Since we are part of one human race inevitably global unity will supersede the artificial barriers which have been created, but this will take time.
And we get visited by aliens from another world. We will then realize there is more to be gained by uniting as one world. England 1000's years ago was divided into several areas who back then would have thought we would unite together but we did further back to the stone age and tribes again who would have imagined today back then. It will happen, only after we know there is more out there
So boring with war, poverty, currency war and crime on earth. No need war for resources, just go to space and mine asteroids.
Plus, I think United Nations (UN) can be the one to lead all of nations together.
If the European Union and USA can do the same why the others can't?
You might think there are too many barriers in the way, for such a thing to happen. You'd be wrong. There are many people of wealth and power working towards this goal. But when it happens, it won't be because of their efforts, or not entirely. It probably won't happen until the Antichrist arrives on the scene. It will happen. Count on it.
During the last one hundred or so years many coalitions have formed, joining multiple countries and/or states together. The best examples are The United States of America and the European Union. The latter continues to grow every year with a total of twenty four countries now, and a recent attempt to add Ukraine to the union. Not to mention the Schengen Area with it's free movement regime.
See, the problem is in order for the nations to be combined into one nation, the others have to surrender their lands either by choice or force. Odds are, no nation will do it by choice if it means they loose their own identity as a nation. The only people who may think this is possible are just assuming that other nations will become part of theirs. If you were the leader of a nation, say your nation, would you hand over control of your nation to a foreign nation? Of course not. This means that the only way to do it would be by force. The problem with doing that is why it has failed in the past. Because you are trying to take over many nations, your troops get spread thinner and thinner. If you take your troops out of an area you defeated, the people there will revolt and take back their nation.
I think the reason why it might seem that the world is coming together is because of globalisation, higher economic dependency, and technological advancement. However, there has never been a higher number of nations as there is today. Many years ago the world had only a few nations, with some having more power than the others, and some trying to create large empires, since the mean to gain power was war. Now, the mean to gain power is economic superiority more than anything else. Thus, countries create unions to gain economic power and defend their own interests. The world has been ever expanding and now we have reached the maximum and have started to have over population problems. So it could be that it is retracting because of that.
I do not believe the world will become a single nation with one government, nor do I believe it will be beneficial for it. As there will be no more competition, which slow technological advancement down, harm the economy of the world, and lower incentives, which means slow inventions, and not progress of the world.