'A mother is the one who is still there when everyone else has deserted you.'
Russia remembered war.
He doesn't wake up screaming, he doesn't have a tremor or PTSD or whatever. Nations have had to face many wars, many traumatic events, so they are made of sturdier stuff, able to move on - or at least, process and understand what has happened to them. They have what humans might call, nerves of steel, although Russia's steel may be a little more cracked than others.
But the blood, pain and distant sounds of death were not on his mind. Instead he remembered something else, in the midst of war. He remembered a song crooned to him as he lay in the cold snow on the Eastern Front, his head pulled onto an impossibly warm lap. He remembered feeling calm, and safe, as arms enclosed him, still singing softly, even though he was on a battlefield. His eyes were bloody so he could barely see, but the distinctive yellow hair and green eyes told him who it was. He remembered England screaming for first aid as the medics swarmed the area, having pushed back the enemy.
It was the first time someone had ever done that for him, and remained the only time. England had not deserted him on the battlefield, even though France was captured and China was gone and America was not coming. England had held him, whispering, "Hold on, they're coming, they're coming, don't go dying on me now," as the medics got closer to their position. He had stayed to see Russia better (which had only taken a few days) before returning to his own land to support the effort there.
Russia supposed England didn't know Russia remembered. He had been drifting in and out of consciousness, half delirious at the time, babbling. But he remembered a slightly rough hand against his cheek, a feathered kiss on his forehead, and England's muttered, "What am I doing? He's not my son, I'm being absolutely mad..." whenever he completed that action.
What he'd do to have that touch again.