Hawaii didn't become Japanese territory after they struck Pearl Harbor.
Guam did, at least until America took it back.
There were something like 4000 colonists killed, but out of how large a population?
Many of the places that the Japanese took over in WWII, the civilian populations far outnumbered the invaders. If Khitomer, and it's population, were under Romulan control following the attack, they would hold the planet as territory, until someone in turn took it from them.
However long that was.