Good ol' Wiki 
"Triage originated in World War I by French doctors treating the battlefield wounded at the aid stations behind the front. Much is owed to the work of Dominique Jean Larrey during the Napoleonic Wars. Until recently, triage results, whether performed by a paramedic or anyone else, were frequently a matter of the 'best guess', as opposed to any real or meaningful assessment.
At its most primitive, those responsible for the removal of the wounded from a battlefield or their care afterwards have divided victims into three categories:
Those who are likely to live, regardless of what care they receive;
Those who are likely to die, regardless of what care they receive;
Those for whom immediate care might make a positive difference in outcome."