[Captain Vlad]

My own personal take on the original question of this thread...

Soldier: A person who serves in a military or paramilitary organization organized by the government of a government entity, is given military training by that organization, and is expected to fight or engage in other combat-related tasks on behalf of that government.

Mercenary: A person, generally one who is, used to be, or has similar skills to a soldier who will act as a soldier for paid compensation.

Assassin: A person who kills or attempts to kill a specific individual for some 'higher' purpose, generally a cause or profit.

Note that things like ideology and loyalty to ones country don't come into play in my definition of 'mercenary' or 'soldier'.

There have been plenty of mercenaries who would really only fight for specific customers, but for whatever reasons did not chose to be involved with that nation's actual military on an official basis. There have also been plenty of mercenaries who were attracted to certain causes, such as the large number of foreign volunteers who fought for both sides in the Spanish Civil War.

As for soldiers...sure, many soldiers have a deep sense of patriotism, but others may have joined up for financial benefit, or to see the world, or because they were drafted. Patriotism is an individual thing, not a defining quality for anyone in a military uniform.