This is one of the military orders players may issue from a nation.

Moving troops is used as a way to spread out troops among friendly nations on the same team. You can send your troops to nations in your empire, or nations not in your empire, but on the same team. All troop movement must occur between nations that are adjacent.

In order to move these troops, you must search for friendly nations first.

Keep in mind you may move more troops then the gaining nation may hold, so to prevent troop loss, do not send troops to a nation that does not have barracks space to store them.

If you are sending troops to a nation in your empire, you can easily check the troop capacity in that nation, but if you are sending troops to a nation not in your empire, make sure you communicate with that nation ruler about the troop capacity that they have.