When something is said to have been done "by force," it usually implies that it was done by actual or threatened violence ("might"), not necessarily by legal authority ("right").
"Force of arms" is a special case that can be an example of unlawful violence or lawful compulsion dependent on who is exercizing the violence (or threat thereof) and their legal right and/or responsibility to do so.
When one citizen threatens another with a weapon, that would typically be an example of the unlawful expression of force of arms. The same threat expressed by a police officer making an arrest would be considered lawful compulsion.