I was thinking about that the other day. In particular I recall a group of frumpy Croat militiamen shouting something along those lines to U.S. Army troops crossing the Sava River into Bosnia, it must have been December 1995.
It seems to be some sort of cynical, jocular greeting that is universal. I would bet that Greeks on the beach at Troy greeted latecomers with it. My son thinks it grows out of dark irony, because over history, few incoming soldiers asked to be there.
My guess is that the salty soldiers say it out of a prideful regret at having experienced war. So I wrote the following:
‘You’ll Be Sorry!’: The Relieved Unit Salutes the Newbies
We have seen what men should not see
We have done what men should not do
And now we know things men should not
(We so wish we still could be like you)