it is often dangerous. >> reporter: and as the families grieve, another question. why was a u.s. soldier left behind on the battlefield? >> the u.s. military does not leave its troops behind and i would just ask that you not question the action to the troops that were caught in the fire fight and question whether or not they did everything they could in order to bring everyone out and once. >> john kelly faced another hard question about the niger mission, which is why are u.s. forces there and this was his answer. he said that u.s. forces there advise and assist nigerian forces so that he, in his words, thousands of u.s. soldiers and marines don't have to go in their place. as you know, that's a strategy that the u.s. has used in afghanistan, libya and many other countries. it's worked in some places, hasn't worked as well in other places here. we lost four soldiers including sergeant la david johnson.