Zee- This is a great question. There is a ton of turmoil that happens well before the existing year of the game in 2103 that helps to shape the Geo-Political scene. In order to spare you the long diatribe of that, I'll simply answer what you asked
All the nations of Earth are still in tact but act more like the states in the United States, or provinces within a given country. In 2090 it was agreed that the Earth needed one unified government with representation from every country and group. This was unanimously agreed and so the United Government of Earth was created. (UGE). It was decided to be sanctioned and established with a large headquarters in Greenwich England.
No one country is above the other. Similarly there are superhero and alien representatives on the UGE council and each country is allowed to govern itself as we see today. It is only when issues in each country start affecting others that the UGE meets to make ruling decisions. Very similar to the United Nations today, but participation in the UGE is total, not inclusive by choice.
All the lessons of the past, all the issues created by the Intergalactic War and arrival of, and habitation by extra-terrestrials, Every scuffle about the right to rule brought up by super and hyper humans has been settled.
To that end Russia and China aren't invading the United States. The UK isn't at war with Somalia, etc. etc. If one country (as implausible as it sounds now in 2103) invades or attacks another country, every other country in the world rallies against the attack and quells it. Peace on a global scale has arisen out of the revelation that we aren't alone in the Universe and the knowledge that there are worse enemies out there in space has made a very humble 'human' brotherhood.
One other small detail to bring this to a close is the Governmental seat of each country. For example there is still a President of the United States (POTUS), but POTUS is also the United States' representative on the UGE. There isn't a separate delegate or Senator etc that is responsible for talking about the U.S.' view on issues. This so critical that the responsibility for being a global view leader is one of the primary campaign strengths that is considered before electing new Presidents