Dulce et decorum est pro patria mori is a line from the Roman lyrical poet Horace's Odes (III.2.13). The line can be roughly translated into English as: "It is sweet and fitting to die for one's country."
The poem from which the line comes exhorts Roman citizens to develop martial prowess such that the enemies of Rome, in particular the Parthians, will be too terrified to resist them. In John Conington's translation, the relevant passage reads:

To suffer hardness with good cheer,
In sternest school of warfare bred,
Our youth should learn; let steed and spear
Make him one day the Parthian's dread;
Cold skies, keen perils, brace his life.
Methinks I see from rampired town
Some battling tyrant's matron wife,
Some maiden, look in terror down,—
“Ah, my dear lord, untrain'd in war!
O tempt not the infuriate mood
Of that fell lion I see! from far
He plunges through a tide of blood!“
What joy, for fatherland to die!
Death's darts e'en flying feet o'ertake,
Nor spare a recreant chivalry,
A back that cowers, or loins that quake.

en.wikipedia.org/wiki/Dulce_et_decorum_est_pro_patria_mori