Ukraine's military leadership has replaced the commander overseeing defenses in the eastern Donetsk region where Russian forces are making swift advances, a military official said on Saturday. General Oleksandr Tarnavskiy, 54, was appointed to head the operational and tactical group Donetsk, replacing General Oleksandr Lutsenko, the official at the General Staff told Reuters. Lutsenko was criticized by Ukrainian military bloggers and some lawmakers for failing to stop Russian troops' relentless push toward the strategic Ukrainian city of Pokrovsk. The battles around Pokrovsk, a key logistic hub for Ukrainian military and civilians in the eastern Donetsk region, come at a critical...