This is an early-ish standalone from Ed McBain that feels quite different from his 87th Precinct series. Published in 1965 (along with 2 87th novels) it's much more of a thriller than a mystery. At 300 pages it's also long for a McBain book. It doesn't feel long though, it moves at a good clip from page 1 and the broad cast of characters serves the story well. McBain had become used to juggling a lot of voices in the detectives and patrolmen of his police procedurals, and he uses those skills here. 
The setting is a small Florida key that gets invaded by a militia group with a grander plan in mind, one that gradually becomes clearer as the plot progresses. The action focuses on both the militia and the local inhabitants, especially the harbourmaster, as they start to fight back. It's tense stuff, with many boating scenes that no doubt draw on McBain's experiences in the Navy. 
Certainly not my favourite of his books, but it's well done, gripping and enjoyably different from the work he's best known for.