FOR SOLDIERS at the front, electromagnetic defences are as vital as air: invisible when present, and disastrous when not. In July Ukrainian troops in southern Donbas found this out the hard way. Abruptly, Russian drones switched frequencies, from standard 700-1,000 megahertz to 400-500 megahertz, blinding Ukraine’s electronic-warfare (ew) systems. The drones flew deep behind the lines, cutting off units and making supply routes impassable. Tens of Ukrainian military vehicles were destroyed daily in what Serhii Beskrestnov, a Ukrainian EW specialist, calls a “Russian safari”. Only when Ukraine understood what was happening, and secured new EW systems working at 500 megahertz, weeks later, were they able to stabilise the situation.