Germany was one of the Great Powers by the time of the Global War. Between the war with Serbia and the war with the Amerasian Alliance, Germany consolidated its position, annexing French colonies, and being extremely harsh on the native populations, using them as slave labour. By 1941, the "Final Solution" was in place, killing 6 million Jews and 5 million political enemies, Freemasons, and disabled, along with countless African natives.
One thing that led to the Amerasian Alliance being formed is that the Black and Jewish populations fled for America, where they hoped they would get better treatment. The Cold War settled in after the US refused to extradite any slaves from Germany.
Eventually, stories of slavery and other atrocities would leak out of the German occupied territories, which would be another reason for the USA forming the Amerasian Alliance.