Some other key events involved Americans coming under attack. The USS Panay was sunk deliberately by Japanese planes in China, and the destroyer Reuben James sunk by German U-boats in the Atlantic. Both of these events brought the US closer to intervention in World War II. Pearl Harbor was the key, though. If you had taken a vote on December 6, 1941, Americans still would likely have voted to stay out of the war. If the same vote were held on December 8th, the day after Pearl Harbor, the result would have been an overwhelming yes.
A more specific question might get an answer that is more useful to you. The most important thing about the progression that you mention is that the US progressed from neutrality to actions that were clearly belligerent well before the Japanese attack on Pearl Harbor. The Germans would have been within their rights to say that the US was acting as a combatant.
The US movement away from neutrality started soon after German actions started WWII. The US gradually came to help Britain and the other allies more and more. This started with trade and moved up to things like the destroyers for bases deal. By the summer of 1941, US involvement had extended to the Lend-Lease program and even to US escorting of convoys across the Atlantic. In this way, the US was doing all that it could short of outright war to help the Allies even before the Japanese attack on Pearl Harbor caused the US to formally become involved in the war.
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