The Plattsburg Manual, an American military handbook published in 1918, contained a section called "Some General Hints", which were words to the wise to be borne in mind on the battlefield. Among them was this bit of advice: "Don't get killed--unless necessary".
Pretty good advice, I suppose!!
Update: A 'little' more context: "Don't get killed unless necessary; your usefulness to the State comes to an end when that occurs." The text of the manual can be found here.