After our violations of neutrality and eventual entrance in WWI, I’m sure Hitler knew it was a foregone conclusion that we’d be in the war.
Yes it was just a matter of time.
In April of 1941 the US Navy was actively escorting convoys as far west as Iceland and depth charging U-boats. We occupied Iceland in May or June of 1941, replacing the British who had occupied in in 1940.
On 31 October 1941, REUBEN JAMES was torpedoed by German submarine U-562. Her magazine exploded, and she sank quickly. 44 of the crew survived, and 115 died. It came out in Congressional testimony that The US navy was actively depth charging U-boats, there as an undercard Naval war at that point.
Given that there was not way the US would not have been able to find an incident to declare war on Germany in December 1941 or January 1942.