If World War III will do, then Tom Clancy's
Red Storm Rising ought to fit the bill. The paper back weighs in at around 750 pages and both the action and overall story are positively gripping. Actually, most of
Clancy's Jack Ryan/John Clark books might your requirements relatively well, although they're generally not "OMG, this is literally the end of the world!!!" stories. However, if you decide to pick them up you may want to stay away from the last three, after he began using "co-writers" (which I assume means the "co" actually writes the books, with a little input from Clancy now and then. At least that's how it feels). In particular
Dead or Alive, which was an epic, epic downer.