這個單字，競爭對手的意味非常濃。兄弟之間競爭，成為對手。所以這句話的意思是“兄弟生為對手”。下面的一段話，解釋了對手和敵人的區別。值得品味。這跟中國文化不同。中國認為兄弟為手足。我沒見到洋大人這樣看。

An adversary is someone you want to defeat. An enemy is someone you have to destroy. With adversaries, compromise is honorable: Today’s adversary could be tomorrow’s ally. With enemies, on the other hand, compromise is appeasement.  http://www.nytimes.com/2013/10/17/opinion/enemies-vs-adversaries.html?_r=0