The natural principle of war is to do the most harm to our enemy with the least harm to ourselves; and this of course is to be effected by stratagem.

The letters brought Irving some early fame and moderate notoriety. His portrait appeared on the bank's notes and contributed to their wide appeal. He made his literary debut in 1802 with a series of observational letters to the Morning Chronicle written under the pseudonym Jonathan Oldstyle.

His historical works include biographies of George Washington Oliver Goldsmith and Muhammad and several histories of 15th-century Spain dealing with subjects such as Christopher Columbus the Moors and the Alhambra. He continued to publish regularly—and almost always successfully—throughout his life and completed a five-volume biography of George Washington just eight months before his death at age 76 in Tarrytown New York.