The Appointments Clause [of Article II] clearly implies a power of the Senate to give advice on and, if it chooses to do so, to consent to a nomination, but it says nothing about how the Senate should go about exercising that power. The text of the Constitution thus leaves the Senate free to exercise that power however it sees fit. Throughout American history, the Senate has frequently – surely, thousands of times – exercised its power over nominations by declining to act on them.
Number 3 Antarctica is the answer
The US borders Mexico, so if Mexico became a part of the axis powers the US would have to fear an attack on its mainland (from either Mexican forces or other axis forces stationed in Mexico), whereas if Mexico remained neutral the US mainland would be safe.
