Trumbo followed the usual course of Stalinist stooges - he hated the Nazis until the Molotov-Ribbentrop Pact at which point he became an ‘isolationist’ against any interventions in Europe. But when Germany invaded Russia he became a fervent warhawk. The US had to go save the Motherland of Socialism, doncha see?
He was absolute scum and, in a just, sane world, would have ended his days on a gallows.
He wrote it as propaganda to convince the American public not to intervene against the Nazis.
Less than a year later, he was gung-ho for intervention because the Nazis had turned on Stalin.
No conscience, no shame.