Emilio Aguinaldo felt betrayed by the United States because, after the Spanish surrendered to American forces in the Philippines, the U.S. did not support the Filipino quest for independence, which Aguinaldo and the Filipino people had expected. Aguinaldo and his allies believed that their fight against Spanish colonial rule would lead to their liberation and the establishment of an independent nation, particularly after they had issued the Philippine Declaration of Independence and set up a provisional government during the conflict.
Instead, the United States negotiated a treaty with Spain that involved "buying" the Philippines for $20 million, and then Congress voted to annex the islands, effectively denying the Filipinos their autonomy. Aguinaldo perceived this as a betrayal of the Filipino struggle for independence, particularly considering that the United States had declared itself as a champion of liberty and self-determination. His resulting call for "war without quarter to the false Americans" reflected his deep sense of betrayal and the feeling that the U.S. had deceived the Filipinos after they had fought alongside American forces against the Spanish.