Navy regularly gets idea to encroach upon others. (6)
I believe the answer is:
invade
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for invade that I've seen before include "Forcibly enter (a country)" , "Penetrate harmfully, country or body" , "Overrun" , "Penetrate other country with armed forces" , "Enter by force so as to conquer" .)