The case of Ex Parte Quirin is of note here, if only to show how inept German sabotage was for the time. A handful of covert German operatives deposited by submarine on the American eastern coast landed in uniform, to comply with the laws of war, buried their uniform, to get around the laws of war, and then started to get busy- only to all be captured within a month or so of their arrival. The issue of whether or not they should be afforded Geneva Conventions is the one the Court takes up in Quirin. Worth your time to read.

The Japanese also launched a series of incendiary hot-air balloons bound for America, but as far as I know, they caused no significant damage to property or persons in the continental United States.