What I did find curious browsing in the RUSI Library, some years ago, was that for at least sixty years up to WW1 it was common to have official military observers missions on both sides, which published volumes of work. One wonders how those missions reported on the Boer War, but more particularly the significant "peer on peer" Russo-Japanese War - which I think signaled many lessons for WW1.

I must look at those volumes one day (if they are still there).

Just as we are being told of a 'foreign fighter' issue, in the limited context of terrorism, it is useful to recall that before 1914 for a hundred years Europeans had fought as private citizens, sometimes liberated countries - Italy, more especially in Latin America. My recollection is that some of this experience was fed back to their home national armies.