What argument is present in the political cartoon to justify imperialism?

The main argument of the cartoon is that white man (in this case United States) needs to educate the uncivilized and undeveloped countries of Latin America. If necessary it will be done even violently.

Explanation:

As the cartoon presents it United States wanted to dominate in the Americas and the only goal to achieve that was to teach the undeveloped regions to accept their guidelines. As we can see, Natives are already pushed in the corner, while Cubans, Puerto Ricans and others would better accept the situation or they will be punished.

it required the unanimous approval of the states before the congress could enact any measure.