Thus, the outbreak of the Second World War was caused by structural economic problems, a "flight into war" that had been imposed by a domestic crisis. The key aspects of the crisis were, according to Mason, a shaky economic recovery being threatened by a rearmament program, which was overwhelming the economy; the Nazi regime's nationalist bluster limited its options. In that way, Mason articulated a "Primat der Innenpolitik" ("primacy of domestic politics") view of the war's origins through the concept of social imperialism. Mason's thesis was in marked contrast to the "Primat der Außenpolitik" ("primacy of foreign politics") by which historians usually explained the war. Mason believed German foreign policy was driven by domestic political considerations and that the start of the war in 1939 was best understood as a “barbaric variant of social imperialism”.