This scene is missing from Decoud's letter, and not described until the next chapter.
Even then, we are not told outright what is meant by their "point of tragic
tension," but we know enough to guess. Antonia will not leave Don Jose's
side, and Decoud must flee, which is tragic enough. But the nature of their parting
holds even greater "tension": Decoud, who despises patriotism, is attempting
to create a new nation for the idealistic Antonia's sake; thus the skeptic has
been thrust into the position of having to literally save idealism.