The novel I am writing takes place in a world where the United Nations became the Global Association of Nations, a union of the entire planet under a collective defensive and political federation. This happens after an alien incursion on earth devastates many of the wealthy nations and essentially sets even the most powerful countries ( United States, Russia, UK, China, etc.) back about a hundred years.
During the alien attack, which was a ground invasion of drones led by a small army of alien soldiers (think the aliens from the movie Battle: Los Angeles), many of the largest and most populated cities on the planet were devastated but not wiped out. Since wiping out cities with WMDs did not benefit the attackers purpose of colonizing the planet for resource extraction, the aliens resorted to extermination using neutron style bombs that would kill humans but leave the cities, and resources, intact.
After several millions are killed by the initial neutron bomb strikes and the subsequent drone invasion, the military forces of the world launch successful counter-attacks and succeed in disabling the drones, thanks to large EMP weapons and destroying the strongholds established by the invaders using tactical nuclear weapons.
In the aftermath of the war, many nations are left devastated. The worst hit are the United States, Canada, the European Union, Russia, China, and other powers in South America and Africa. However, governments and their military wings aren't wiped out and do recover. Nations begin rebuilding.
However, in the immediate decades following the incursion, the nations of the world are concerned that another invasion may follow in the future. With the world's economy also devastated, political unrest follows and many are concerned that opportunists (be it terrorists or rouge nation states) will hinder if not endanger recovery. As a result, first proposed by surviving world leaders of the EU, and later supported by Brazil, China and other powers, the nations form the G.A.N. at a historic United Nations emergency assembly in Geneva, Switzerland, uniting their portions of their military services into a collective armed forces, a peacekeeping organization, tasked with restoring order and defending their planet from another attack while safeguarding the nations recovering from enemies both foreign and domestic. However, even after the decade since the G.A.N.'s formation and nearly 20 years after the incursion and independent national reconstruction began, problems are still present.
While some would say the shear devastation and the fact much of the infrastructure remains non-functional, the biggest is the ethnic and political divides that still exist and threaten the collective armed forces initiative. As stated before, native peoples would have a hard time dealing with the idea of a political organization outside their own domestic government trying to tell their government how to run itself. Nor would they like a foreign soldier acting under global law trying to enforce said global law in their land.
With that being said, my question is, given the situation, how would that armed forces be organized overcoming the political and ethnic divides?