By Jonathan Landay FRONT LINES NORTH OF KHERSON, Ukraine (Reuters) – Ukrainian troops at the front line said on Wednesday they were expecting a bitter fight for the southern Kherson region, which Russia now appeared to be reinforcing after days in which it seemed possible that Moscow might withdraw. In Russia, the military staged a high-profile rehearsal for nuclear war, with state television broadcasts dominated by footage of submarines, strategic bombers and missile forces practicing launches in retaliation for an atomic attack. Moscow has conducted a diplomatic campaign this week to promote…