"Come Immediately"

Poul Anderson, The Shield Of Time (New York, 1991).

Of the four Exaltationists at large in 209 BC:

two, in the city of Bactra, have instruments that will detect the arrival of a temporal vehicle anywhere nearby and will flee into space-time if they suspect that the Time Patrol is approaching;

the two others, guarded by several soldiers, meet in the temple of Poseidon outside the city.

Everard of the Patrol, armed with a sword, is in the temple but cannot overcome two Exaltationists and their armed guard. However, one of the Exaltationists wears as a medallion a captured Patrol communicator. Everard kills him, seriously injures the other and, as the soldiers attack him, says into the communicator:

"'Unattached Everard. Come immediately. Combat.'" (p. 113)

Then it is over. Patrol agents on five timecycles arrive a moment later and stun everyone in the temple except Everard who then leads them in an attack on the Exaltationists in the city sixty seconds earlier, before any instruments can have detected the arriving timecycles. Without breaking their own rule against changing the past, the Patrollers use time travel to win a battle as soon as it starts.