Counterfactual US history (fiction) where a private firm develops the first nuclear bomb & Texas secedes

[Andy]

About 20 years ago, I read a book from a university library, which was an alternative history or counterfactual. It was a hardcover edition, maybe from the 1960s or 1970s.

The only details I remember are these:
- it was astonishingly detailed; for example the author wrote an appendix with the results of regional elections, spanning decades (again, all fictional);

- one key point was that the US government had not built the atomic bomb in WWII. Rather, the first atomic weapons were designed and built by a private company, which was a massive multinational conglomerate that figured centrally throughout the book;

- The state of Texas either seceded or somehow became/remained part of Mexico, and was an independent republic for a long period of history.

That's all I remember. I contacted the library where I saw the book, but they couldn't find it, and had no additional information for me.

Thanks.