ONIS, Luis de (o'-nees), Spanish diplomatist, born in La Mancha, Spain, in 1769; died in Madrid about 1830. He was trained in the diplomatic service, employed in the state ministry when the French invaded the peninsula in 1809, and is believed to be the author of the famous manifest of Ceballo. The supreme junta of Cadiz appointed him envoy to the United States, but, notwithstanding his repeated efforts, President Madison refused him recognition, under the pretext that, as the crown of Spain was in dispute, the American government could not pronounce in favor of either of the belligerents. Onis, however, remained in this country, where he rendered great service to his government by communicating events in Spanish America and transmitting orders to the governors and commanders in those countries. After the return of Ferdinand VII. to Spain in 1814, Onis applied again for recognition to Washington, and it was finally given in December, 1815. He renewed his former protests against the American occupation of Pensacola. Mobile, and part of Florida, and against the fitting out of expeditions for the Independents of South America in Baltimore and other parts of the Union, but obtained, however, only evasive answers, and in order to avoid further complications, seeing no hope for the recovery of Florida, he used his best efforts to bring about an advantageous treaty for its cession to the United States, the abandonment of the American claims for indemnity, and the fixing of the boundary between Louisiana and Texas. The treaty was signed in Washington in February, 1819, and Onis returned to Spain to hasten its ratification by his government, which was delayed by court intrigues till 1821. Onis meanwhile had been appointed minister to Naples and afterward to England, whence he was recalled in 1823, when the absolute power of King Ferdinand was re-established. He published in 1810-'12 in the United States, under the pen-name of "Verus," satirical letters, in which he attacked the conduct of the United States government toward Spain; and he also wrote "Memorias sobre las negociaciones entre Espafia y los Estados Unidos de America, que causaron el tratado de 1819; conteniendo una estadistica del ultimo pais" (Madrid, 1820; English translation, with notes by Tobias Watkins, Baltimore, 1821).