German spies landed in the United States in a (failed) attempt to cause harm to our economy.
Operation Pastorius was a plan to land teams of German spies (all of whom had lived in the United States) by submarine off the east coast of the United States in June 1942. The spies were equipped with false birth certificates, Social Security cards, $175,000 in counterfeit money, and dressed in German Navy uniforms so they wouldn't be executed as illegal combatants. They were equipped with bombs and ordered to meet up and attack important American economic targets and cause terror on a two-year mission.
The plan nearly backfired when the first team of four spies that landed on June 12 off New York City was discovered by a Coast Guard patrol after they landed, but the spies slipped away and weren't caught. The second four-man team landed safely in Florida on June 16. Unfortunately, the head of the operation (Dasch) secretly hated the Nazis and betrayed his comrades. The eight spies were arrested; Dasch was sentenced to 30 years in prison, another, Ernst Burger, (the spy he confessed to) received life in prison, and the others were all sentenced to death.
Sources:
Insidious Foes: The Axis Fifth Column and the American Home Front p. 131, by Francis MacDonnell
Shadow Enemies: Hitler's Secret Terrorist Plot Against the United States, by Alex Abella and Scott Gordon
George John Dasch and the Nazi Saboteurs
German submarine U-202
http://uboat.net/boats/u202.htm
German submarine U-584