Might not be the earliest,but my favorite is in July 1941 during the US occupation of Iceland. OTL Hitler was annoyed to discover the US intended to established a naval base in the midst of the North Atlantic war zone. However the invasion of the USSR at the end of June had his attention & the US action was adjusted to. The PoD would be in June when intelligence reports informed Hitler of the US intent. Upset he has a tantrum and orders the Navy to teach the US a lesson about entering a war zone. Some submarines are positioned to interdict the US warships and convoy carrying the base personal and Marines. Ignoring the German diplomatic warnings the US Navy runs the submarine gauntlet & a couple warships or transports are torpedoed. At least one is sunk. This doubles down on the ongoing US/German naval battles in the Atlantic. While a DoW may not occur that week the Isolationist OTL had become to weak to prevent this escalation. Hitlers worst move at this point would be to authorize the submarines to enter the Western Atlantic Exclusion Zone and attack cargo shipping there. Congress is liable to recognize the reality and remove opposition to 'defense measures' the USN undertakes. With a actual US/German naval war underway Hitler orders war declared on the US.
Note: Several times earlier in 1940 & 1941 Hitler had discussed if a DoW on the US would then be useful. He backed off each time short of a DoW. Given his haphazard nature & shaky political and strategic logic its not ASB for one of these earlier incidents to bring on a DoW.
Note: The USN had expanded Amphibious Forces Atlantic Fleet in early 1941. A full strength Marine Division and the Army 1st Inf Div corps formed the landing force corps. They and the Atlantic Fleet were updating and rehearsing among other things the Occupation of the Portuguese Azores (War Plan GRAY) should Spain become a German ally, or be invaded by Germany.
Note: There was a faction among Spanish leaders who considered independent Portugal a aberration and had plans in place for invading that nation at the earliest opportunity. Had Spain thrown in with the Axis in 1940-41 such a conquest of Portugal might occur. Contrary to a canard, Salazars government was not in deep sympathy with the Facist parties ruling many other European nations. Had there been a threat from Spain, Germany, or Italy the intent was to invoke Portugals treaty of alliance with Britain and remove the government to the Azores.
Had Spain been occupied by Axis soldiers, either in alliance or conquest it sets the situation up for confrontation between the US and the Axis if the US executes its plans to occupy the Azores and establish naval bases there.