By Karin Strohecker (Reuters) -The United States and its allies have vowed to hit Russia with harsh sanctions after Russian forces invaded Ukraine on Thursday in a massed assault by land, sea and air – the biggest by one state against another in Europe since World War Two. Western capitals had introduced an initial round of sanctions after Russian President Vladimir Putin recognised two breakaway regions in eastern Ukraine on Monday, though vowed to ramp up measures following the latest events. British Prime Minister Boris Johnson unveiled a package of “severe” sanctions against Russia, target…