Sure, but what is "highly trained". I used to think that the Marines were some sort of super soliders for instance, them apparently having undegone the toughest training, being sifted through inhuman testing and then seasoned with sheer torture(I mean, so Hollywood says). And that would perhaps be true, for maybe one out of thirty of the ones serving. At least the ones participating in the joint winter excercise when I served were like that. Their teams were made up like this: They had one sergeant, he looked like he had surv.. served on every tour of duty since vietnam, but the rest of the soliders in the team were pitifully green as fresh grass. Judging from how they handled themselves, they were even greener than we were at that time after six months service. In fact they did so many strange things I thought at first that they were some kind of reserve force sent on vacation to "the land of the midnight sun" or something similar. But these were in fact Mercenaries, I was told quite forcefully actually by one of these sergeants, and the mainstay of the US forces. Apparently, they were on their way to Bosnia. They could shout real good, though.

I don't like to picture these guys under fire, to be honest. Perhaps they learned fast then. In any case, the point is that unless you're deciding to pick out all the ones with combat experience for a mission, which is difficult after a while, you'll end up having rookies with you. And they will, contrary to what some think, after one year of training at the most be fit for handling a billion dollar accident with sprinklers. That's just how it is. Or, they could be grown in vats and fitted with synthetic memory... nah.
Being homesick? If you asked me, I would say the reason why there has been so few "friendly fire incidents" is that most of the fresh forces are on guard duty. Only if you asked me, of course.