It’s a chief principle in military affairs to go where ye are least expected.

Robert Louis Stevenson (1850–1894) Scottish essayist, novelist, poet
Novel (1886), Kidnapped, ch. 26 “End of the Flight: We Pass the Forth” [Alan]
    (Source)

This chapter was originally published in Young Folks Paper, Vol. 29, No. 816 (1886-07-17).

See Sun-Tzu 1, 2 (5th C BC).