Soviet Union support. Agreement for cooperation was signed in 1919: Soviet Union got army training, technologies (wasn’t officially recognized until 1934 so no other significant sources) and help with initial industry development; Germany got resources (metals, food, oil, coal etc.), built dozens military factories on the Soviet territory to bypass Treaty of Versailles (research and production) and received facilities to train tank crews, pilots and officers. They cooperated later in partition of Europe. And even when later Nazi were out of bombs to attack London, USSR gave bombs too.
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