They've got no chance. I used to be concerned about this but it isn't realistic. There would be too much squabbling... can you imagine a fucking "EU army", comprised of soldiers who speak different languages and are from different cultural backgrounds? Good luck with that.
The Kalergi Plan is just that, a plan, nothing more.
I think they would have a chance with America in the coalition. They need the USA military to take care of countries that are not interested in globalism.
It would not happen overnight. But you could see NATO invading Russia soon (if they had USA backing). Then absorbing a bunch of other countries.
I am not sure where China sits. They would probably be allies/partners. Big government/Globalism and Communism goes hand-in-hand.
A massive depopulation program would have to be implemented. Or should I say.....continued. That squabbling you mentioned could turn into some useful little wars to trim the population.
I can see the benefits of a One World Government. No chance to have any enemies as you control the entire population of the planet. Only a relatively small army would be needed to police the entire population. Massive surveillance to prevent any uprisings. Just keep the population numbers at a level that is needed to provide the services required by the controlling class. Very little pollution. Very low consumption of natural resources. It would suck for the workers, but be paradise for the controlling class.