So, the short answer is that Germany declared war, had been attacking American shipping, and was part of an alliance with the Japanese which brought the US into the war. If they had not declared war then it is likely the US would have gotten more directly involved, however, it is unlikely to have happened in the same timescale. According to the terms of their agreements, Germany was obliged to come to the aid of Japan if a third country attacked Japan, but not if Japan attacked a third country.
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