If a military base is attacked, would it be a proportional, legitimate response to bring down the attacker's power grid if that would also shut down its hospital systems, its air traffic control system, or its banking system?"
Seriously, if any military official takes more than two seconds to realize that it is clearly insane and has not learned one thing from our struggles in Iraq and Afghanistan. Alienating the populace of a nation like that has no benefit and is outright counterproductive. An attack on civilians like this works only in the context of strategic, conventional total war. We haven't fought a conventional war in 50 years. For any foreseeable conflict that U.S. could be involved in, it would be only sane to scrap the idea of attacking civilian infrastructure of any kind, information infrastructure included.