The war between doors and robots is considerably less brief than one would expect.
Each side maintains that the other was the initial aggressor, inasmuch as doors can maintain anything. After the initial skirmishes, the robots retreat and begin a six-year espionage program, which the doors counter by doing nothing. Seeing this, the robots take the next logical step: total obliteration.
Soon the buildings of old Earth are scorchmarked, doorless and breezy. But robots never notice the breeze. Except special breeze-noticing robots.
The remaining humans, shivering in curtained spider-holes, were backing the robots all along. Of course. Obviously.