The story
Attributed to various military leaders, including Bernard Montgomery, Dwight Eisenhower and Douglas MacArthur, the quotation has existed in various forms since the Second World War. It was popularized when used by the character Vizzini in the 1987 movie The Princess Bride (after appearing in the 1973 novel The Princess Bride). Asia is far more heavily populated than Europe or North America, and while Western armies had technological advantages post-WWII, these could be overcome (as they were in Korea by the Chinese PLA and Vietnam by the PAVN) by sheer numbers.
Etymology from Wiktionary, CC BY-SA 4.0.