The Anti-Imperialist League argued that imperialism and expansionism went against the spirit of the United States' founding documents, such as the
Constitution. Which event from the late 19th century or early 20th century
most clearly supports this argument?

The American annexation of the Philippines.

Explanation:

The Anti Imperialist League was created in 1898, in response to the annexation of the Philippines by the United States. It was, basically, a group of conservative democrats who opposed the expansionist idea once defended by Manifest Destiny, as well as the militarization and strategic expansion ordered by the main leaders of the Republican Party. Its foundation lay in the principle of freedom and consensus of the governed, by means of which they said that America would act as a tyranny to try to expand its territories, by crushing the rights of the inhabitants of those lands.

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