Why might a relatively long coastal land get destroyed a lot but not captured?

[Foxkeyes]

You could introduce a supernatural element. Perhaps this is some 'sleeping' force that awakens whenever new conquerors appear and change things. The 'entity' is tied to the original inhabitants. It drives away the conquerors. Not immediately, but over time. Thus slowly keeping the status quo.

 

[Aldarion]

I want a land to go the way of the Phoenix; defeated often but rising again, usually with new people, each time, in its history. I'm already stuck with it being a long stretch of coastal land with a few important natural harbors with a climate like Stockholm. I want to know how this could plausibly be so; why doesn't the defeating power of the day come in and take the land itself? Current era is industrial revolution. I want the conditions to be the case so that the land is self-sufficient enough to ape Prussian militarism at the end of the 19th century. I'm not committed to any other resource/geographical features.

If defeating powers are pirates, they may be strong enough to raid the coast and devastate settlements along it, but not strong enough to conquer it permanently... much like Muslim (Berber) Corsairs were from 7th to 19th centuries in the Mediterranean.