Bradford Snell's testimony to Congress in 1974 addressed this question (link, pp. 16-23). The Snell report to Congress maintained that US senior executives continued to exercise virtually full control over Opel’s operations for the first 11 months of declared war between Germany and the US. However, GM and other researchers have rebutted this claim vigorously. GM claimed that the “enemy property custodian” appointed by the German authorities had sole authority over the management and operations of Opel. Billstein et al take a nuanced view of the question in Working for the Enemy. They take issue with Snell's assessment that GM exercised "complete management control" at Opel (35), but they argue that Opel executives and managers continued the general strategies preferred by GM before the war. And they argue that US executives of GM during the pre-war years were eager to gain contracts for vehicles and other materials that were crucial for the Nazi government's military buildup. "In fact, the evidence suggests General Motors's willing collaboration in the conversion [to armaments production]" (36).
Billstein et al raise a crucial and foundational question: could GM have vetoed the conversion of Opel’s manufacturing capacity to wartime production in the 1930s and the use of forced labor in the 1940s if they had wished to do so? Henry Ashby Turner, Jr.'s General Motors and the Nazis: The Struggle for Control of Opel, Europe’s Biggest Carmaker sheds more light on the business activities and decision-making of General Motors during the Nazi regime. Turner has special expertise on this question, since he directed the documentation project during 1999-2000 sponsored by General Motors to review its private corporate archives during the Nazi period. Turner has special authority in his judgments about GM's wartime behavior because of his direct involvement in the 1999-2000 review of General Motors' internal documents and records during the relevant time period. Further, Turner is an acknowledged expert on the corporation's behavior during the period. (The complete archive of all documents recovered and reviewed has been deposited in digital form at Yale University' Sterling Memorial Library, designated as the General Motors-Opel Collection.) Based on systematic review of massive quantities of internal GM documents in 1999-2000, Turner concludes that GM's management control over Opel was extremely limited after 1941.