He knows what an exploding TARDIS looks like. There were many TARDISes in the war, all crammed with soldiers. But an exploding TARDIS is like nothing on Earth. It is thousands of times more powerful than a hydrogen bomb, capable of cracking the universe.
Maybe that was one reason the war lasted so long. Time took the power and used it to resurrect countless soldiers. Countless times a TARDIS could explode, only prolonging its agony.
He knows what an exploding TARDIS looks like—a great ball of fire larger than any black hole, large enough to warm a whole planet.
