what is the 'truth untold' that the soldier speaks of in Owen's "Strange Meeting"

The "truth untold" refers to how alike both enemies are on the battlefield. It alludes to the paradox of "the enemy". People seem to thing that the killing on a battlefield exists in a vacuum. The truth Owen wants to point out is that the young men killing each other are more alike than their own countrymen at home. They feel the same horrors, futility and shattered dreams; they are enemies and they are brothers.