Read more of our recent coverage of the Ukraine warSince late January Russia’s army has been hurling troops and shellfire along wide swathes of the front lines in the east. One scenario is a repeat of last summer, when Russia’s conquest of Severodonetsk, a town in Luhansk, came at such a high price in casualties that Ukraine was able to punch through Russia’s thinned-out lines in Kharkiv two months later. As Russian military power approaches its nadir, Ukraine’s task is to make the most of an opportunity that might not come around again