The Mexican Revolution was as confusing as described by Riley and Cornelius. The roles of Villa, Zapata, Huerta, Carranza and Obregón were pretty much as stated. Patton was indeed Pershing’s brother-in-law and also his aide during the Punitive Expedition. Villa suffered a serious gunshot wound to his right leg at the battle of Guerrero on March 27, 1916. Accounts of his whereabouts during his convalescence vary, but it is said that American soldiers led by Lieutenant Summer Williams almost captured him at the home of a supporter named Rodriguez. Williams became suspicious when he saw Yaquis near the home, as it was known that Villa’s men included Yaquis. There is also a legend that Villa recuperated from his wound in a desert cave.
Germany certainly plotted to start hostilities between the United States and Mexico, hoping to involve the United States in the Mexican Revolution and thereby delay its entry into World War I. This became known through the Zimmerman telegram, mentioned in passing in the novel, in which the German Foreign Secretary spoke of offering Mexico support for a war against the United States and promised the return to Mexico of land in Texas, New Mexico and Arizona. British Intelligence obtained the telegram and its publication is said to have hastened America’s entry into the war.
I have found no mention in history of Otto von Kleist, which is perhaps not surprising given that he was a top-secret agent and a highly unsavory member of a distinguished family. Modern psychology would likely classify him as a sociopathic sexual sadist, but I expect Riley and Cornelius would have a different diagnosis: he was a monster.
Pershing never found Villa, but somebody finally did. He was cut down by rifle bullets from hidden assassins while riding in a car on July 20, 1923. The identities of his killers are unknown.