David O. McKay said,

There are, however, two conditions which may justify a truly Christian man to enter–mind you, I say enter, not begin–a war: (1) An attempt to dominate and to deprive another of his free agency, and (2) Loyalty to his country. Possibly there is a third… Defense of a weak nation that is being unjustly crushed by a strong, ruthless one…

(David O. McKay, CR, April 1942, p. 72)