Dalton Trumbo, like many of the blacklisted film and literary figures, was anti-war, at a time when anti-war meant anti-American. He wasn't a Communist sympathizer in that he supported the regime, but rather sympathized with Russian people who would be the victims of an all out war with the US.
This quote from Trumbo following WWII in 1946 is especially relevant today for the people of any country fearing annihilation in a mutually assured destruction scenario. For example, cross out Russia and replace it with America.
"If I were a Russian...I would be alarmed, and I would petition my government to take measures at once against what would seem an almost certain blow aimed at my existence. This is how it must appear in Russia today"
Dalton Trumbo supported the Soviet regime in the 30's and railed against the Nazi regime as he should have. Then when the Nazi's and Soviets signed the nonaggression pact in 1939, he wrote and spoke about the glories of the Fuhrer until the Germans broke the pact in 1941. Believe me, I've read up on Mr. Trumbo on this subject. Coincidentally, he wrote his anti-war novel, Johnny Got His Gun, during this period.