The United States found itself in an advantageous position at the beginning of the second world war. The nation was still reeling from the effects of the great depression and was still in desperate need of financial assistance.
They certainly had no interest in directly joining the fight over in Europe but, they were very much interested in the financial opportunity the war provided them. They would get to enjoy the benefits of neutrality while making industrial contributions to their European allies. By that I mean that they became an ally supplier in the first years of the war. They sent over weaponry, military vehicles, and medical supplies.
Naturally this chain of buying and selling supplies was kept secret (as much as reasonably possible) from the axis powers as it could have been seen as a direct interference in the war and would likely have caused axis powers such as Germany to openly declare war on the United States, effectively ending their neutral status. Does this mean that the axis powers had no idea about this trade? No, various accounts and physical reports would suggest that the axis had suspected the United States of this long before the US broke their neutrality.
However, the axis already had their hands full with managing the European war front as well as maintaining the loose relationship with Russia and were therefore less than eager to invite the US to join the fight by any means. As far as the axis powers were concerned the US was an issue to be dealt with at a later date and thus the US was able to maintain neutrality while providing military aide to the allies.