"The spies have defected."
This seems like a case where -ta' would usually work well. Somebody usually wouldn't defect by happenstance. I would think that generally quite a bit of forethought and plotting would be involved. I have probably watched Red October way too many times.
-pu' doesn't imply happenstance. -pu' is the general perfective suffix; -ta' is the perfective suffix specifically relating to intentional accomplishment. You can always use -pu', even where -ta' would also be appropriate.
I hit the enemy (whether on purpose or not).
I hit the enemy (on purpose).