In context: http://suntzusaid.com/book/4

I think the quote is an alternative translation of paragraph 15 in the link above:

"Thus it is that in war the victorious strategist only seeks battle after the victory has been won, whereas he who is destined to defeat first fights and afterwards looks for victory."

It has an associated commentary:

Ho Shih thus expounds the paradox: "In warfare, first lay plans which will ensure victory, and then lead your army to battle; if you will not begin with stratagem but rely on brute strength alone, victory will no longer be assured."