The Russians successfully hacked the US government in 1996 using a remarkable ploy recounted in a recent story in The New Yorker.
To break into the American military's network — which was classified and not connected to the public internet — the Russians planted bugged thumb drives in kiosks near NATO headquarters in Kabul, hoping that an American serviceman or servicewoman would buy a drive and plug it into a secure computer.
Pretty smart, and it shows how we have to be careful of IT devices.