By Andrew Osborn and Polina Nikolskaya MOSCOW (Reuters) -President Vladimir Putin authorised “a special military operation” against Ukraine on Thursday morning to eliminate what he called a serious threat, saying his aim was to demilitarise Russia’s southern neighbour. In an early morning address on state television, Putin said he had been left with no choice but to launch the operation, the scope of which was not immediately clear but appeared to go well beyond helping Russian-backed separatists in eastern Ukraine. “I have decided to conduct a special military operation,” said Putin, seated a…