Historically, leading war is an essential part of Nazi German foreign politics. While you could lower your neutrality low enough to go to war as early as the end of '37, you should consider waiting not only to 1939, but to 1940. In 1940, the democratic powers outside the allies (mainly the United States) have an election. If you hold back aggression, support the German-American Bund via spies long enough and diplomatically influence the US, this industrial super power could join the Axis, practically granting you victory as no one is there to stop Japan and the allies miss important troops on D-Day, on the Invasion of Italy or in the North African war. The following strategies will, if not stated otherwise, an allied USA from 1941 onwards. Note the US could join a bit sooner, a bit later or not at all. This mainly depends on Japan.
==Invasion of Poland 1939==