If you want to, I think you can invade Russia without declaring war, because the Korean war is still unresolved, and North Korea's military is active in Russia.

If you want to, I think you can invade Russia without declaring war, because the Korean war is still unresolved, and North Korea's military is active in Russia. Just claim your advisors to South Korea are taking care of extra-territorial combatants from that war. No reason to declare a war on Russia when it's clearly part of the existing conflict.