Courage, mettle, spirit, resolution, tenacity are comparable when they mean a quality of mind or temperament which makes one resist temptation to give way in the face of opposition, danger, or hardship.
Courage stresses firmness of mind or purpose and a casting aside of fear (for courage meaning courageousness, see courageous under BRAVE); it implies a summoning of all one’s powers in order that one’s desires or ends may be achieved.
Mettle suggests an ingrained or characteristic capacity for meeting strain or stress in a manner suggestive of a finely tempered sword blade.
It often implies qualities (as resiliency, ardor, fearlessness, fortitude, or gallantry associated less with physical strength than with mental or spiritual vigor.
Spirit, like mettle, refers to a temperamental quality but suggests something more volatile or fragile. It implies an ability to hold one’s own, to assert oneself or one’s principles, or to keep up one’s morale when opposed, interfered with, frustrated, or tempted.
Resolution, like courage, implies firmness of mind and purpose, but it stresses determination to achieve one’s ends in spite of opposition or interference of men or of circumstances rather than a casting aside of fear of danger or a dread of hardship.
Tenacity adds to resolution the implications of stubborn persistence and of unwillingness to acknowledge defeat.