Ukraine has opened a new front in its battle to drive out the Russian invader - in Russia. But it is oddly coy about admitting that it has sent troops, fired artillery, and flown drones into its neighbor’s territory. The operations of Russian citizens, carrying Ukrainian military ID, wearing Ukrainian uniforms and attacking from Ukraine, remain officially opaque. It is Kyiv’s contribution to what’s become known as “hybrid warfare” in the “grey zone” of contemporary conflict. The two terms provoked books and a tsunami of excited opinion from an army of pundits when Russia first invaded Ukraine in 2014. Back...