He promptly joined the army as a medical officer with the rank of major, and sailed for the islands on the second steamer which carried United States troops there.

As a natural result of his familiarity with Spanish and his wide acquaintanceship among the Filipinos, he was ordered from the outset to devote his time more largely to political matters than to the practice of his profession.

He did all that he could to prevent misunderstandings between Filipinos and Americans.

He assisted as an interpreter at the negotiations for the surrender of Manila on August 13, 1898, after taking part in the attack on the city.

Later he was given the rather difficult task of suppressing a bad outbreak of smallpox among the Spanish prisoners of war, which he performed with great success.