US President Donald Trump has fired Mark Esper as the defence secretary and announced an acting replacement, making the announcement over Twitter.The step was not entirely unexpected and comes in the wake of Trump losing the presidential election, although he continues to contest the results.There have been simmering tensions between the two since the summer, with Esper having publicly distanced himself from Trump over the tough crackdown by the federal government targetting protests against racism and police violence.The president had considered using a law from 1807 to deploy military forces…