We've (excluding the US,) been at peace for some time, and cables are getting cut left and right. Not even all recently, even 15 years back there were some suspicious ones out by Asia, forget the details, but Fox was up in arms about it.
Edit: What should be done, is to mass produce some gear to hook onto the cables or near them, to spy out sabotage attempts and give early warnings. Then scramble the warplanes.
Or even an electrical line, with flashpoints where if a diver or submarine drone gets near, they can send electricity through the water to incapacitate the threat.