By Natalia Zinets and Aleksandar Vasovic KYIV/MARIUPOL, Ukraine (Reuters) – Ukrainian forces battled Russian invaders on three sides on Thursday after Moscow mounted an assault by land, sea and air in the biggest attack on a European state since World War Two. After Russian President Vladimir Putin declared war in a pre-dawn televised address, explosions and gunfire were heard throughout the morning in Kyiv, a city of 3 million people. Missiles rained down on Ukrainian targets and authorities reported columns of troops pouring across Ukraine’s borders from Russia and Belarus to the north and e…