By Natalia Zinets and Aleksandar Vasovic KYIV/MARIUPOL, Ukraine (Reuters) -Ukrainian forces battled Russian invaders on three sides on Thursday after Moscow unleashed the biggest attack on a European state since World War Two, prompting tens of thousands of people to flee their homes. After Russian President Vladimir Putin declared war in a pre-dawn televised address, explosions and gunfire were heard throughout the day in Kyiv, a city of 3 million people. The assault brought a calamitous end to weeks of fruitless diplomatic efforts by Western leaders to avert war. “This is a premeditated atta…