León María Ignacio Agapito Guerrero y Francisco, GCrM KGCR (March 24, 1915 – June 24, 1982), better known simply as Leon Ma. Guerrero III, was a Filipino diplomat and novelist, and was one of the foremost Filipino nationalists of his era. A partner in the law practice of senator Claro M. Recto, he became Undersecretary of Foreign Affairs during the Magsaysay administration. His then controversial advocacy of Asia for the Asians and espousal of a realistic re-examination of relations with the United States are now commonly accepted as being ahead of their time.

Leoni, as his family called him, was born on 24 March 1915 in Ermita, Manila to one of the nation's most prominent families.^{[1] [2][3] [4]} His parents were Dr. Alfredo León Guerrero and Filomena Francisco, the first Filipino woman pharmacist. His sister, Carmen Guerrero Nakpil, would also be grow up to be a journalist and one of the most preeminent figures in Philippine letters.

His paternal grandfather was León María Guerrero y Leogardo, a distinguished botanist, Malolos Congress delegate and member of the first Philippine Assembly. His maternal grandfather was Gabriel Beato Francisco, a journalist who had been manager of El Comercio, the foremost mercantile newspaper during the Spanish regime.

During enrolment for high school, Guerrero met Horacio de la Costa, who was hailing from Tayabas, Quezon. The two of them would become very close friends later on. In his high school days, Leoni had studied under the Classical Latin Programme as well as English literature and composition, algebra, American history and civics, Philippine history and government, economics, religion, public speaking (including debate and military drills) and physical science.^[5] Due to his meritorious achievements, his mother would put his medals in a box and safekept it in an aparador.

It was through the Guidon, the student publication of the Ateneo de Manila University, that Leoni, Horacio de la Costa and Jesus A. Paredes, Jr. had cemented their lifelong friendship. Initially, the three of them were staffs, but later on became the publication's editors.^[6]

In 1938, he married Anita Corominas of Cebu. She was the daughter of Don Jose Corominas and Doña Paz Escaño. Her maternal grandparents were Don Fernando Escaño and Doña Agustina Faelnar. Her niece was married to the son of José E. Romero, the first Philippine ambassador to the Court of St. James's.

In 1954, Guerrero was sworn in as Undersecretary of Foreign Affairs by then-President Ramon Magsaysay and Vice President-elect Carlos P. Garcia. He, also succeeded Felipe Neri as the president's special adviser in foreign affairs.^[7]

On retirement he was the country's senior career diplomat, having served as ambassador in London, Madrid, New Delhi, Mexico City and Belgrade. On June 19, 1982, only a few days before he died, he received the, Gawad Mabini, the highest award in the Philippine Foreign Service.

He held the rank of Knight Grand Cross of the Knights of Rizal. Among his many works are internationally acclaimed translations of José Rizal's Noli Me Tángere and El filibusterismo. He also wrote a biography of Rizal titled The First Filipino.

• Twilight in Tokyo (1946)
• Passion and Death of the USAFFE (1947)
• Report from Europe (1951)
• Alternatives for Asians (1957)
• An Asian on Asia (1958)
• The First Filipino (1962) (Awarded first prize in the Rizal Biography Contest under the auspices of the José Rizal National Centennial Commission.)
• El Si y El No. (1963) (Winner of the Premio Zobel)
• Las Dos Muertes de General Aguinaldo (1964)
• Two Friars in Exodus (A Contribution to Studies in Philippine Church History) (1969)
• The Philippine Revolution by Apolinario Mabini (1969)
• Today Began Yesterday (1975)
• We Filipinos (1984) (Posthumous anthology of his writings)

• Noli Me Tangere, by José Rizal (1961)
• El Filibusterismo, by José Rizal (1962)

• : Grand Cross (Dakilang Kamanong) of the Gawad Mabini, 1982^[8]
• : The Order of the Knights of Rizal, Knight Grand Cross of Rizal (KGCR).^[9]