According to the TNG episode "The Defector", the Norkan Outposts were destroyed in a massacre by the Romulans.
Now I admit to knowing very little about stardates and the entire TOS series timelines, but my questions are these:
Did the Norkan Outposts belong to the Federation?
And if so...
When was the Federation formally at war with the Romulan Star Empire?
Technically, the Federation has been in a cold war (with periodic bursts of actual hostilities) with the Romulans since its inception, but in TOS' "Balance of Terror," it established that Earth and Romulus went to war during the 22nd-Century (presumably a few years prior to the Federation's formation).
The Norkan Outposts were Federation outposts destroyed by the Romulans during a series of border skirmishes along the Neutral Zone possibly in the early 24th-Century and prior to the Romulans entering their period of isolationism, IMO.