Of course he thought the US was a military threat.....in 1944!

Seriously though, prior to WW 2 the US ranked 36th as a military power worldwide. In the First World War, the US got in too late to really have a psychological impact like it did in the Second. Additionally, Hitler, and most of his inner circle, had hardly ever been outside Germany, let alone travelled to America.
I remember reading commentaries by captured German soldiers sent to POW camps in the US (typically places like Arizona). They repeatedly say their opinion of Germany winning the war changed to one that they were doomed as "day after day we travelled on a train across America and saw fields full of tractors and machines, automobiles everywhere, factories, and towns and cities untouched by war."
Hitler's view, and it seems reinforced by commentaries like Göring's "All America can build are refrigerators and razor blades!," was that the US was little or no threat. It was half-a-world away, had a worthlessly small military, and no means to project power to Europe or anywhere else. No one had ever seen a nation prosecute a war around the world fighting on fronts thousands of miles from home. I think that the concept of this even being possible was beyond Hitler's comprehension when he declaired war on the US.