What if the world was all just one country. rather than multiple countries?

I asked a few of my friends about this, and they all gave me mixed responses. Some in favor, and some against. Considering that this doesn't cost a war, or any form of violence, Could it be a good idea? It would solve any issues with fuel, but would also make it so all country's had accessible roads, and thus, more cars. And how would it affect the pollution/global warming issues?