Seventy years ago Wednesday, four German saboteurs — armed and trained for a mission of destruction — slipped ashore in Amagansett. Though a minor footnote in the annals of World War II, it was one of the very few known incidents in which enemy operatives set foot on United States soil. Moreover, in recent times, the military tribunals in which the would-be attackers were tried and sentenced have been cited as the legal antecedents of how cases are handled of prisoners at Guantanamo Bay, Cuba.