There would certainly be war, states which are federally governed would fear for their identity, this would lead to conflicts.

Then there are the different currencies, school systems, languages, dialects, cultures, country characteristics, health systems, product standards, etc.

Using my country as an example, I can explain it as follows:

We Swiss would vote on it and even if we were allowed to remain federal (with referendums and autonomy of cantons/provinces, we would always vote NO,. Then add to that, if we were to join with federalism, the others would also be allowed to be federally governed, if no, there would be disagreements > war.

If such a thing is possible, then only like Starfleet in the Star Trek universe, after the end time mood, when everyone has the same goal and the same rights.