One of my favorite movies (and others here have said the same) is
Kelly's Heroes.
One of the themes of the movie is the "fighting spirit" of those soldiers involved in the operation. The motive is a share of the pile of gold which is the objective.
Everyone is motivated by self interest. For some self interest is money, for others it is Honor, Duty, Country, etc., for almost all it is self preservation when the bullets fly.
Perhaps the best soldiers will be those who have all of these as motivation.
It hasn't been unusual for Mercenaries choose the cause for which they will fight, and a wise commander will seek those for his command, and be aware of which lesser Mercenaries have been enlisted.
Fighting for Self Interest, in any combination, is the Free Market (Libertarian) way
