He lists several reasons: the static, defensive nature of the war that prevented either side from delivering a decisive, victorious blow to the other; the powerful political position of the military within the combatant countries' governments; the fact that the participants were industrial powers with large populations that could sustain their war efforts; the fact that both sides succumbed to the 'sunk costs' fallacy ("the more each side lost, the more it had to promise to deliver once victory was achieved"); and the ever-increasing territorial ambitions of the various combatants.
Walt also points to the role of propaganda and censorship in sustaining the war in spite of its unspeakable carnage:
A negotiated settlement was never seriously attempted, in part because censorship and wartime propaganda convinced citizens on both sides that victory was just around the corner. Tight military censorship ensured that populations back home got an overly upbeat picture of how the fighting was going, with reports from the front tending to omit bad news, portray defeats as victories, and offer upbeat assessments of future progress. As Prime Minister Lloyd George told a friend in 1916, "If the people really knew [the truth], the war would be stopped tomorrow. But of course they don't know and can't know."Walt argues that the lessons of World War I resonate today, especially as it relates to censorship, propaganda and the demonization of the enemy:
Furthermore, wartime propaganda portrayed the enemy as brutal monsters guilty of vast atrocities, and these malign images of the enemy hardened as the number of dead and wounded increased. How could politicians seriously entertain negotiating peace with the vicious opponents who were busily killing off the nation's youth? Meanwhile, governments boosted public support by portraying the war as a noble crusade; in England, for instance, thousands of copies of poems like John McCrae's "In Flanders Fields" were distributed to encourage the population not to "break faith with us who die." In this atmosphere, anyone who seriously proposed negotiating an end to the fighting -- as Lord Lansdowne in England or historian Hans Delbrück in Germany did -- was quickly denounced as a traitor who was undermining morale at home.
[T]he long and bitter experience of World War I reminds that truth is the "first casualty" in war, and gleaning accurate information about how a war is going is extremely difficult. Soldiers have a natural tendency to tell superiors what the latter want to hear, commanders will spin upbeat stories to maintain popular support so that they have time to deliver a victory, and the media are easily co-opted by their own feelings of patriotism and by sophisticated media management strategies (such as "embedding").
This problem continues to bedevil us today: just look at all the upbeat reports of progress that we heard about Iraq or Afghanistan since 2002, and look at where both countries are now. Or ask yourself whether the "war on terror" is going well or not, and whether the vast sums spent on "dirty wars" in Yemen, Somalia, South Asia, or the Middle East been worth it. Today, as in World War I, the people paying for these wars, and providing the sons and daughters to fight them, are kept mostly in the dark about whether we are winning or losing.
Third, the tendency to demonize and dehumanize the enemy remains a central feature of modern warfare, just as it was during the Great War. Today, Hamas offers up anti-Semitic and conspiratorial depictions of its Israeli oppressors, while Israelis increasingly embrace racist depictions of Palestinians. Sunni and Shia continue to kill each other over a doctrinal dispute dating back to the seventh century. Muslims and Hindus attack each other in India and elsewhere. If you believe you might have to kill a large number of foreigners, it helps to convince yourself that they aren't fully human. But World War I warns that treating enemies as if they are subhuman beasts only makes the conflict last longer, because politicians cannot compromise with a hated foe and many will think it is foolhardy even to try. And the longer a war lasts, the less likely it is that any of the warring parties will end up better off.