President Donald Trump has pardoned four Blackwater security guards who were given lengthy prison sentences for killing 14 civilians in Baghdad in 2007, a massacre that caused international uproar over the use of private contractors in war zones. The four – Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten – were part of an armoured convoy that opened fire indiscriminately with machine-guns and grenade launchers on a crowd of unarmed people in the Iraqi capital. Known as the Nisour Square massacre, the slaughter was seen as a low point in the conflict in Iraq. In 2014, Slough, Liberty and...