短语讲解：

1. Mr. Obama had announced plans to cut back the troops numbers to 5,500 by the end of 2015.

by the end of 到…为止
例句：By the end of July I'll have read all those books. 
到七月底前，我会读完那些书。 

2. The soldiers fired on the protesters, killing at least one.

at least 至少
例句：For that day at least he lost heart. 
至少这一天他是垂头丧气的。