The Bizarre Plan The US Had To Invade Portugal

In the early 20th century, the United States had a number of color-coded contingency plans in case war broke out with another country. For example, War Plan Orange was for Japan, Black for Germany, Red (and its various hues) for Britain and its commonwealth territories including Canada, Gold for France and its territories, and even Indigo for invading Iceland in the event of defending the Eastern Seaboard (via We Are The Mighty). Seemingly one of the most arbitrary was War Plan Gray, the plan for an invasion of Portugal developed in 1941. 

At this point in time, Portugal was an officially neutral country, and though it did export materials to both sides it never made substantial contributions to the Axis. Francoist Spain by contrast, while officially neutral as well, supplied a volunteer force for the German invasion of the Soviet Union and the two were close allies during the Spanish Civil War. According to the plan's details, America didn't necessarily fear Portugal voluntarily becoming an Axis power, but rather that a particular territory of theirs might fall into German hands (via Global Security). 

The Azores could have been the key to German air raids over America

The Azores are a nine-island archipelago in the Atlantic Ocean that have been Portuguese territory since their discovery in 1427 (via Britannica). The islands are situated nearly 1,000 miles west of Europe, and during World War II, both American and German strategic planners recognized they would be more than useful to the latter as both a U-boat base and as an airfield to bomb the American East Coast (via Warfare History Network). Germany had several prototypes for an "Amerikabomber," but none could carry sufficient fuel for a two-way trip. 

However, if these aircraft took off from the Azores, the fuel spent would be greatly reduced. For the same reason that Britain invaded the then-Danish territory of Iceland per Spectator, the Americans planned to invade the Azores in case Portugal was invaded by the Axis. War Plan Gray was in fact only the first plan for such an event, and it was developed further into the joint-Allied Operation Alacrity. While an invasion would have probably been successful, none ever occurred and in any case Portugal consented to Allied airbases being built there and within its own borders by 1943 (via The New York Times).