网上看到的一句话：
The top U.S. commander in Afghanistan had been out campaigning for more troops on top of the 21,000 that Obama had approved shortly after taking office.
其中的 had been out campaigning for more troops on top of the 21,000，没弄明白所表达的意思？因为：had been out（原型是be out吧？）不知作何解，与其后的 campaigning …之间的结构关系没见过。
敬请专家予以说明！
如题