Final answer:
Imperialism became more popular in the United States due to economic interests like seeking new markets and raw materials, ideological beliefs in American exceptionalism, and strategic considerations. The desire to keep pace with European powers and the consequences of the Spanish-American War also influenced this shift in attitude towards imperialism.
Step-by-step explanation:
The rise in popularity of imperialism in the United States by the end of the 19th century can be attributed to a variety of factors. Among the reasons, economic motives, such as the desire to sell goods to new markets and secure sources of raw materials for burgeoning businesses, played a significant role. This economic drive was exacerbated by the fears of falling behind industrial powers of Europe and the competition for global influence and resources.
Americans also supported imperialism due to a belief in 'American exceptionalism', which promoted the spread of democracy and Christian values internationally. Additionally, imperialist sentiment was fueled by strategic and military considerations, as gaining control of territories enhanced the United States' geopolitical power, provided military advantages, and secured trade routes.
The ideological battle at home was intense, with the Anti-Imperialist League forming in 1898 to oppose expansionism, citing democratic principles, economic concerns, and racial anxieties. Nevertheless, the Spanish-American War and subsequent events, such as the Filipino insurrection, swayed opinion towards formalizing imperial gains, eventually leading to the ratification of treaties that expanded American sovereignty over new territories.