At 2 a.m. on April 30, 1919, a postal worker named Charles Caplan was heading home to Harlem on the subway. It was the end of a long night’s shift, and Mr. Caplan, who worked in the city’s main post office, at 30th Street and Eighth Avenue, was reading — of all things — a newspaper article on a mail bomb that had recently exploded at the home of Thomas Hardwick, a Democratic senator from Georgia. Something caught his eye.  It was a close description of the small brown package in which the bomb had been delivered, and it immediately caused Mr. Caplan, a clerk in parcel post, to think about the 16 similar small brown packages that at that moment were sitting in his storeroom back in Midtown. At the next station, he left the train and hurried back to work: he arrived in time to catch the deadly parcels and, in so doing, forestalled what could have been — and surely would have been — one of the worst serial bombings in the country’s history.  At a moment when domestic terror — whether from American jihadists or from other homegrown groups — seems particularly threatening, we may do well to remember Charles Caplan, who, not unlike today’s observant citizens, immediately notified the relevant authorities: in his case, the defunct (and incomparably named) New York City Bureau of Combustibles. A quick investigation determined that the packages contained what were known at the time as “infernal machines” (think, instead, incendiary devices.) But far more disconcerting were the addressees: J. P. Morgan Jr.; John D. Rockefeller; Mayor John F. Hylan; Richard Enright, the police commissioner; and a host of judges, politicians and prominent business figures from Washington, D.C., to the California coast.  Within days, a stash of high explosives was discovered in a house on 45th Street, and “undesirables” from as far away as the Pacific Coast were quickly rounded up.  “Radicals Watched in Bomb Plot Hunt,” read a front-page headline in The New York Times a few days after the scheme had been discovered. The lead? “There are more than 2,000 radical agitators in New York City who have been preaching Bolshevism and the overthrow of the United States government, and every one of these persons is now under investigation by federal and local authorities.”  In the end, those authorities would blame the plot on an Italian insurrectionist named Carlo Valdinoci who, in fact, would die the following month when one of his successful bombs — with him a bit too close — exploded outside the home of A. Mitchell Palmer, the United States attorney general.  With the public stirred into a fury, Mr. Palmer and his colleagues in the government reacted with an iron investigation: warrantless surveillance, scrutiny of subscriptions to radical publications, and eventually the arrest and deportation of scores of subversives that came to be known as the Palmer Raids.  It was as if a local cell of Al Qaeda had organized a big one, arranging — and almost carrying off — the delivery of letter bombs from a secret New York safe house to the likes of Mayor Michael R. Bloomberg, Police Commissioner Raymond W. Kelly, George Soros, Rupert Murdoch, and Sonia Sotomayor, to boot.  Can you imagine the reaction? Perhaps the overreaction?  Then again, the Palmer Raids, despite their obvious brutality, appeared to work. Where are the anarchists now?
In 1919, a postal clerk saved New York City from attack via mail bombs, otherwise known as "infernal machines."