The way Gatiss delivers “What do you mean, one?” breaks my heart.
Because they really did think that the Great War would end wars–in fact, they called it the War to End All Wars, but in the end it wasn’t. In the end it allowed for humans to invent increasingly innovative ways to murder each other. (There’s a reason that Mordor is ash and mud and thick choking smoke, a no-man’s-land of churned earth where nothing grows, where Sam and Frodo miserably slog through a ruined and blasted landscape.)
And, in the end, the Great War’s resolution directly sowed seeds that would ripen into World War II. (And on and on and on. Humans solve one war and lay the foundation for the next; we simply seem hellbent on waging war on each other over anything and everything.)
And Twelve’s response is to apologize, to say ‘Spoilers’, and he looks downright glum about even momentarily pricking the bubble of hope this World War I soldier has. And then One goes “Enough of this” and we see character juxtapositions already.