National government
Earth was ruled by nations for thousands of years, from the river-valley theocracies through the great empires to the nuclear-age powers.
For thousands of years Earth was governed by nations, each with its own government, borders and army: theocracies in the river valleys first, then the empires and monarchies of Eurasia and the Americas, then the great powers of the industrial and nuclear centuries. That arrangement is over. As Mars was settled and the damage to Earth's environment grew past anything a single country could address, the national governments first became subordinate to the United Nations in practice and then ceded their sovereignty to it outright. What is left is one government over the whole species, with no nations underneath it.
How far into The Expanse have you read?
Set your place and this page rewrites itself to exactly that point. Nothing ahead of you will show.