"Dulce et decorum est pro patria mori "

Dulce et decorum est pro patria mori is a line from Horace's Odes.  It 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 so that the enemies of Rome will be too terrified to resist them.

The line was famously reproduced in Wilfred Owen's poem, Dulce et Decorum Est, which describes a gas attack during World War I.  Owen uses the line in the final lines of his poem, describing it as "the old lie."