 The number of slave laborers grew as the war expanded although Wallace makes it clear that companies in Germany were not required by the Nazi authorities to use slave laborers.When Rolls-Royce sought a U.S. manufacturer as an additional source for the Merlin engine (as fitted to Spitfire and Hurricane fighters), Ford first agreed to do so and then reneged.He "lined up behind the war effort" when the U.S. entered in December 1941.