re: It's been part of tactical military planning for decades, both for offense and defense.
But no effort has gone into using it as a strategic weapon, or defending against it.
Not entirely true. There have been programs, most of them classified, that have addressed the threat. One project was a big one that dates back to the early 90s and worked on shielding fighters, comm-sats, Mil-link and OCOC vehicles in situ from EMF effects. Facilities have been built completely impervious to 99% of electromagnetic field penetration.
But are these technologies being shared with the private sector? No way. Will they be? No way.
But if need be...they could be.
If a serious threat registers as credible to the total infrastructure, I imagine the right people in the right places would work to share some of the technology in a timely manner to preserve the US infrastructure.