War in 1939 followed escalation and miscalculation

Did Germany start the war in 1939 because it needed an external conflict to prevent civil war?

No; Hitler expected Britain and France to back down and miscalculated the consequences of escalating against Poland.

The answer rejects a theory that invasion in 1939 was necessary to prevent civil war inside Germany. Hitler did not expect his move against Poland to trigger the major war that followed; he believed Britain and France would back down as they had during earlier escalations. When they did not, Germany gambled on a short conflict because it lacked the resources for a long one. Opening another war against the Soviet Union before defeating Britain compounded that strategic error. The sequence presents the war's timing as the result of escalating risk and failed assumptions, not an unavoidable domestic timetable.