The Royal Marines motto “per mare per terram” (“by sea, by land”) could have been used to signify Prince Harry's role as
Captain General of the Royal Marines, a post he inherited from Prince Philip.
However, it is a military custom in much of the US armed forces to sign off letters, speeches and even individual social interactions with a unit motto.
This custom is not practised in the British armed forces.
Oh dear...How odd that Prince Harry signed off his comments in the American fashion! Its almost as if someone American wrote the comments for him!