[4] He was sent by his superiors in the Nachrichten-Abteilung, the military intelligence arm of the German Imperial Admiralty Staff, to the still neutral United States in 1915, at age 38, on a false Swiss passport under the name of Emil V. Gasche (the surname appropriated from his brother-in-law).

Arriving in New York City, he posed as businessman Frederick Hansen and with Heinrich Albert, who from 1914–17, served as Handelsattaché (commercial attaché) at the German embassy in Washington D.C. set up a dummy corporation called Bridgeport Projectile Company, through which they made large purchases of gunpowder, which was then destroyed.

[10] During 1915, he negotiated with Victoriano Huerta for money to purchase weapons and U-boat landings to provide support, as Germany was hoping to persuade Mexico to make war on the U.S. through the Plan of San Diego, as this would divert the United States military and all munitions supplies towards defending America's southern border.

Also in 1915 he bought ammunition and supplied money to the deposed Mexican dictator Huerta and encouraged him to try to seize back power in Mexico.

[16] After the liner was diverted to the United Kingdom he was arrested at Southampton, but protested his innocence so skillfully that the Swiss Embassy and Scotland Yard were both persuaded.

[5] He was then extradited to the United States, tried and found guilty in a Federal court in New York, and imprisoned in Atlanta, Georgia, for three years, after the U.S. entered the war.

According to some sources, Captain von Rintelen despised the Nazi Party so intensely that, during World War II, he willingly taught the Special Operations Executive how to construct and use all of his former bombs and incendiary devices.