First, hi and thank you for a wonderful new toy. I am still exploring, but slowly getting into the swing of things.
OK, this is a quasi humorous exploration of a legal premise that may not exist in game.
I started in the Pennsylvania (PA) system, found a quick triangle trade route, made a fair amount of starter cash, then went wandering to New York (NY).
After an abortive attempt to go NPC bounty hunting, I ended up at West Point. Finding Marines as a cargo, I filled up on them to "transport" to PA to drop at the battleship.
So, my (amusing) thought is, is it legal to transport Marines from one nation to another? Since they are not being used for direct military action, it should not constitute an act of war, but, what about the sale of "arms" to a foreign power?
Pennsylvania and new York are both liberty controlled, you weren't transporting them between opposing nations. Even if you were, maybe you were acting as an unaligned infiltration tool
Welcome to Discovery server. At first, I must warn you. You can find illustrative truthful warning about how Disco works here
So, you have been warned...
Now back to topic. It is legal with one exception (see below). But... Marines are cargo with roleplay potential so it very depends on many factors what will happen. For example your ID, where you will ship it and also your and other players interaction.
In Bretonia it is restricted cargo and you need license to transport it, otherwise you are in trouble. In other houses, most of time nobody will care. But for example if lawfuls stop you in Kusari, ask you about who these marines are and you answer they are private gunmen/bounty hunters, you can get into problems because PMCs are not welcome there.
Or another example, if you will be stopped in Hamburg flying from the Liberty, or in Texas flying from Rheinland (these two houses are currently at war...), you can face accusation that you are carrying enemy soldiers and thus you have to be arrested or at least cargo must be confiscated. Because this is exactly situation when simple transporting marines from one nation to another doesn´t make any sense because Libertonian marines really would not be welcome in Rheinland and vice versa.