Now it is a good time to do so. As of 1.15.4 (Gotterdammerung), you need not to worry about casualties sustained, just capitulate France (do not create Vicky France) and the UK, but do not capitulate Poland. In the peace deal, annex all Polish and continental French states plus Corsica, then the achievement is completed. Germany has enough military forces to easily crush the Allied in the 1939 scenario.
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