Incidents arousing anti-Spain sentiment in the United States included:
the explosion of the Maine
anti-Spain newspaper stories
distribution of propaganda against Spain
a letter from the Spanish ambassador
the signing of the Monroe Doctrine

a letter from the Spanish ambassador

Explanation:

On 9 February 1898, Spanish Ambassador Enrique Dupuy de Lome wrote a letter to Spanish Foreign Minister Don Jose Canalezas, expressing De Laum view of Spanish involvement in Cuba and US President McKinley's diplomacy. In the letter, Spanish Ambassador Enrique Dupuy de Lome criticized US President William McKinley, urging the audience to favour only the weak and agitated. The words included in the publicly seized Spanish letter caused an international uproar, which contributed to anti-Spanish and anti-war sentiment in the United States. The publication of the letter brought public support to the Spanish colony Cuba in its war against Spain over independence.