Russian troops launched a wide-ranging attack on Ukraine on Thursday, as President Vladimir Putin cast aside international condemnation and sanctions and warned other countries that any attempt to interfere would lead to “consequences you have never seen”. Big explosions were heard before dawn in Kyiv, Kharkiv and Odesa as world leaders decried the start of a military operation that could cause massive casualties, topple Ukraine’s democratically elected government and threaten the post-Cold War balance on the continent.