ZAPORIZHZHIA REGION, Ukraine — The path to a Ukrainian victory — or at least the most obvious path — will probably cut south, through the muddy and flat fields of the Zaporizhzhia region. Following Russia’s retreat from the city of Kherson — the only regional capital captured by Moscow since the start of Russian President Vladimir Putin’s invasion — Ukrainian forces have limited options for their next big push to continue recapturing occupied territory and, ultimately, to expel the invaders. Much attention is now shifting here, to the southern front line less than 100 miles north of the Azov Sea,...