Lest one thinks that war only affects law enforcement on a national level, the local NYPD was heavily prepared for imminent involvement in the World War I:

“…[for] invasion of bombardment or the cutting of supplies by siege, detailed answers to every phrase of that question are on file at Police Headquarters… Plans for emptying the town or any given section of the town are perfected. In addition to all the normal traffic lines, elevated, surface and subway, which the Police Commissioner has the right to commander if the lives of the people are at stake, he he on record lists of many thousands of motor trucks and other vehicles which would be at the instant disposal of the police…

By means of this same Police Department with its emergency plans the entire food and fuel supply of the city could be municipalized overnight and its distribution regulated by the authorities in the way to do the most good for the largest number.”

One would assume that the NYPD in 2017 has an even more complex plan for such an unlikely scenario, especially following a massive terrorist attack on New York City soil in 2001.

There was also a warning to avoid paranoia or overly broad measures in 1917, in stark contrast to the “Muslim registry” advocated during the campaign trail by our current president:

“This suggests another thing the ideal policeman has to be. He must be part diplomat. Nothing would be more absurd or fraught with danger of serious consequences in a time like this than for the police to act on the assumption that all Germans are suspects. There are 300,000, or more, of them in this city. The occasional plots of the last two years and a half in this country and city would indicate that perhaps a very few of these Germans have to be watched — probably not one in a thousand.”

New York Police Department Is on a War Footing: Nothing Has Been Left to Chance During the Last Two Years to Prepare 11,000 Policemen for Any Emergency to Come

From Sunday, February 11, 1917