[Spoliers!] ###########
It's been a while since I've read the book, but if I recall correctly nukes are used against Venezuela when they disagree with launching supplies to the Ark project. There are further hints of looming conflict (e.g. Ivy's fiance mentioning the nuclear sub he was in was on high alert).
I also believe there were numerous delays of supplies and disagreements even within NASA / the U.S. space program. Not too certain of all the world powers cooperating, although admittedly their cooperation on the Ark project at all may seem odd, but doesn't seem out of place with the status quo and how world powers "get along" in regards to the ISS?