这个单字，竞争对手的意味非常浓。兄弟之间竞争，成为对手。所以这句话的意思是“兄弟生为对手”。下面的一段话，解释了对手和敌人的区别。值得品味。这跟中国文化不同。中国认为兄弟为手足。我没见到洋大人这样看。

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