AS IRAN, Israel and America trade blows in the Middle East, ghost armies, guns-for-hire and terrorist proxies have become a powerful arm of war. Mercenaries can be found fighting across the world – from Russia’s shady Wagner fighters to bloodthirsty Colombian veterans known as the Desert Wolves. From the Congo jungles to the deserts of Iraq and the Ukrainian frontline, mercenaries have reshaped modern warfare. Made up of professional soldiers who fight in the shadows of battlefields, these highly-trained fighters are available to whoever can afford to hire them. Unrestrained by rules of traditional warfare, so-called “rental armies” often have...