I saw This in /r/PropagandaPosters. Was there a credible threat of nazi spies during the war or where they just being overly cautious?
Not really. Nazis did not have a effective spy network in the US or UK. The UK caught all of the spies the Germans sent and either turned them into double agents, or imprisoned them. The British then helped the US to round up spies and encouraged the US to build their own double agent network. The United States was more interested in killing captured spies. There was a U-Boat the dropped off 3-4 spies on Long Island during the WWII. Those men had all lived in New York City and moved back to Germany just before the war. They all spent one night drinking with old friends and then turned themselves into the Federal Government. They were all executed.
Sources: [Double Cross by Ben Macintyre] (http://www.nytimes.com/2012/09/02/books/review/double-cross-by-ben-macintyre.html?_r=0)
Not really, here is a wiki article about the Duquesne spy ring and here's one about a failed submarine landing attempt. While the SS had it's own intelligence agency, the Abwehr was the main one for everyone else, and it's higher ups were actively anti-Nazi, so anything to do with them was bound to fail, here's some info on them.
