Final answer:
The sentence in question underscores Theodore Roosevelt's stance that engaging in strenuous experiences like war is necessary to maintain national vitality and masculine vigor, echoing his advocacy for a 'martial ethic' and the virtue of facing challenges.
Step-by-step explanation:
The quoted sentence from the student's question appears to develop Theodore Roosevelt's claim about the significance of strenuous, formidable experiences in shaping national character and individual masculinity. Roosevelt argues that engaging in challenging, even dangerous endeavors - such as war - is vital for national vitality and the cultivation of a 'martial ethic', which fosters manliness and prevents societal decadence. The sentence speaks to Roosevelt's belief in the importance of 'great and generous emotion', 'high pride', 'stern belief', and 'lofty enthusiasm', portraying these as quintessential qualities for those who face and conquer great challenges. This rhetoric is aimed at stirring a sense of urgency and responsibility among men to embrace hardship and conflict for the betterment of the nation and to assert their dominance on the world stage.