The Underdog always wins - 哀兵必胜

哀(ai)兵(bing)必(bi)胜(sheng)

"哀" means "hopeless, depressed"
"兵" means"arms,soldiers,military"
"必" means"must"
"胜" means"win,triumph"

So it means"an army burning with righteous indignation is bound to win".

Corresponding English idiom: "The Underdog always wins"

Example sentece: Nick unexpectedly beat Kaka from China in the table tennis match, perhaps the "underdog always wins" which is often said. (尼克竟然在乒乓球比赛中赢了身为中国人的卡卡，这或许就是常说的哀兵必胜吧.)