...and Gandalf suddenly had a brilliant idea of how to win the war.

[HerenIstarion]

Matthew 7:22-23:

22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? 23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.

Still, casting out devils remains the good cause. It is means (iniquity) which is condemned, regardless the goodness of the cause. If Tolkien were to elaborate the point, though, I suppose he would not approve of Aragorn and Co involving in such activities.

In fact, he haven't appoved at all, had he? - otherwise, we would have seen ork hirelings in published LoTR, would not we?