"As the angry aliens converged on the landing party, Kirk wondered how the hell things had gotten so out of control . . . ."
Although I'm talking less about seguing into a flashback and more about beginning the book with a flash-forward teaser.
In either case, my instinct would to be include a "Twelve Hours Later" header anyway, just in the interests of clarity and avoiding confusion. Sometimes the brute force approach is just what the job calls for. You don't want to be so subtle you confuse the reader . . . .