Final answer:
Concerns about the integration of other races and the fear that empire building was antithetical to American democratic values posed internal threats to nationalism in the United States. The rise of nativism and the Red Scare period exemplify how internal strife and fear of foreign ideologies undermined national unity.
Step-by-step explanation:
The threat to nationalism in the United States during the era in question was multifaceted, encompassing internal and external concerns. Among the options provided, concerns about the integration of other races (Option C) and concerns that empire building ran counter to American democratic principles (Option D) directly relate to internal nationalistic tensions, while fears of competition from foreign workers (Option A) and the idea of a foreign invasion (Option B) deal with external threats.
Focusing on internal aspects, the rise of nativism and fear of foreign ideologies such as communism during and after the Red Scare period represent a significant threat to U.S. nationalism. The internal strive for ethnic homogeneity and the backlash against internationalism, which ran counter to the principles of pluralism and inclusivity that are part of American democratic ideals, deeply challenged the nation's self-conception.
The early 20th century, marked by the Red Scare and concerns over communist infiltration, shows how the paranoia and distrust towards immigrants and certain ideologies can threaten the fabric of national unity and pride, thus posing a threat to the nationalism that held a diverse society together.