Propaganda was used globally during World War One, urging people to join the fight, encourage hatred against national ememies, and moreover, convince the public how just the cause of War was. Propaganda as a whole exudes patriotism/nationalism, placing emphasis on how enlisting was their duty, and that fighting would bring them glory and honour--a story that had been told for hundereds of years. But this was an illusion; the heroism of war was nothing compared to its shocking reality. Following WWI, people were disillusioned, and this is what led to the rise of Modernism. This image is a British propaganda banner, urging men to protect their country, mothers and daughters from the "horrors of war". It's ironic how they mention these horrors, while sending their men off to face these very same horrors. The final lines of "Dulce Et Decorum Est" by Wilfred Owen reads, "Dulce et decorum est, Pro patria mori" (Lines 27-28), which translates to "It is sweet and fitting to die for the homeland." Owen, who experienced the horrors of war firsthand (and unfortunately died shortly before armistice), calls this phrase an "old lie" (Line 27), completely denouncing it.
Sources:
https://www.poetryfoundation.org/poems/46560/dulce-et-decorum-est