When I grew up (in the UK), one thing that was pushed a lot in the general ambience of discussion about world war 1 were what I know are misconceptions and disservices – e.g. Blackadder 4, which I love, but is obviously not historically accurate. Before asking this, I did some reading and found that a lot of the origins of many common takes are from the 60s (Lions led by Donkeys coming from a book by British MP Alan Clark, who later admitted to lying in the book).
My question is not about if the “Butcher Haig/disregard for the lives of the soldiers/etc” reputation is true or justified, but rather about the origin of that public perception and the political forces behind that emergence.
A lot of the myths and misconceptions that Clark wrote about in 1961 were actually well established in the literature of the war by that point. The depiction of Haig and his generals as a pack of stolid, unimaginative blunderers who wasted lives in their arrogance is one that dates back to the war itself, but really took root in the interwar period. I'd say that by the start of WW2 the tone for much of the public perception about WW1 was pretty firmly in place.
It was in the 20s and 30s that most of the books of what we now often call the "battle of the memoirs" were published. Winston Churchill published his five volume series The World Crisis between 1923 and 1931. Wartime prime minister David Lloyd George's six volume War Memoirs came out between 1933 and 1936. Military leaders were also busy protecting their reputations, such as General Hubert Gough who published The Fifth Army a defence of his record in 1931, General Horace Smith-Dorrien or Field Marshal John French. All of these works were partisan efforts to either bloster one's own reputation or to denigrate an opponent's. Lloyd George's memoirs are probably the clearest example of this, aiming to blame Haig and GHQ for the immense human cost of the war.
But alongside the memoirs of leaders at the highest level, there were also the books by journalists and other veterans. Philip Gibbs was an official war correspondent and once freed of wartime censorship he published The Realities of War in 1920. This was a very critical attack on Haig, GHQ and the generals of the British army, describing them as "intellectually, with few exceptions, narrowly moulded to the same type, strangely limited in their range of ideas and qualities of character." Gibbs went on to say a lot more unsavoury things about generals, based very firmly in his own experience and frustration.
And lastly there's the influence of one Basil Lidell-Hart. Liddell-Hart was a veteran of the war who'd served as a junior officer until being badly gassed on the Somme in 1916. Liddell-Hart began writing on military topics during the war and became a journalist and military correspondent after the war. He wrote several influential books on generals and wars and then in 1931 published The Real War (1914–1918). The book was another scathing attack on Haig and British generalship during the war, but also doubled as an introduction to Liddell-Harts own military theories. His own views were largely in line with critics of Haig like Gibbs and Lloyd George. The later influence of Liddell-Hart after WW2 gave the book subsequent importance in the post-war period that it probably didn't have in the 1930s. However, Liddell-Hart didn't just write books, he also wrote to people. A lot. His papers, held by Kings College, run to 10.57 cubic metres or 537 boxes and 720 volumes of papers. He helped or advised many others on their own works, two important for this conversation being both Lloyd George and Alan Clark, and maintained correspondence with just about every military historian in Britain, including a long friendship with the editor of the British Official History of the War James Edmonds, many important political and military leaders and many others. He would have public disputes, particularly over Western Front campaigns, in the editorial pages of the Times that could go on for months. Through this very wide network and his considerable influence (I read a letter from the 30s referring to Liddell-Hart as "the other Chief of the Imperial General Staff") that Liddell-Hart was able to keep this view dominant. One of the cheif opponents of Liddell-Hart towards the end was John Terraine, probably the most well known defender of Haig and one of the first who took a somewhat revisionist approach to WW1 history. After Liddell-Hart died in 1970 more revisionist histories began appearing, heralding a new scholarly interest in the war, taking a less partisan approach.
There is absolutely another wave of this sentiment from the 60s onwards, but it's less scholarship and more in the realms of entertainment that these ideas gain traction. Things like Oh What a Lovely War, The Monocled Mutineer and the ascension of the War Poets that had widespread cultural impact. I have heard it argued that it was the addition of the poetry of Sasson and Graves to the school curriculum more than anything else that drove this particular view of the war, but I haven't seen any work to back that up.