From an older answer with some minor tweaks:

This isn't an easy question to answer, and offering a truly broad overview of “veterans”, even in a single country let along many of them, is a fairly gargantuan task which I suspect has at least in part been why this remains unanswered. I’m not going to take such a Macro view here, but instead focus narrowly, quite narrowly in fact, on one veteran, although a rather prominent one.

Sgt. Alvin York was a Medal of Honor winner in “The Great War”, and certainly one of the most famous - if not the simply best known - American soldier of that conflict in the years after. His heroics, single-handily taking on a massive amount of Germans, not only made for a stirring story, but his biography - a one-time conscientious objector turned war hero - only added to the aura.

Two decades later, as the Second World War broke out and America initially stood on the sidelines, York proved to be a fairly vocal proponent for increased American involvement. Even prior to the beginning of “World War II”, York had shown himself to be a stirring voice against isolationism, and of the potential for American military power to be a guarantor of world peace. In 1937, as the Second Sino-Japanese War was already seeing hundreds of thousands of casualties in Asia, he rather insightfully noted the possibility that continued Japanese incursions would draw America into the war. Not only was he of the belief that the US needed to stand “ready to fight again in the interest of peace”, but even that the US ought to be more forceful and proactive - “I’d just as soon get into it now as later”. He also wasn’t sparing Hitler in his views, although initially less forceful about actual American intervention. Commenting to reporters at a memorial event held just in the wake of the Munich Agreement, he showed no illusions that Hitler would remain satisfied, opining that “If we want to stop Hitler, we must knock him off the block”. A few months later, as the world continued to inch closer to war, he told reports that the United States had a duty to rearm, and “ought too be prepared to defend the Western Hemisphere”, and although he expressed optimism that conflict might still be averted, he felt America needed a strong military in the event war did break out, for a bristling armed neutrality.

The outbreak of war in Europe, of course, only bolstered his views, giving truth to his prophecies, and shifting his mind from a mere necessity to defend America against potential invasion to the possible need to return 'over there'. In the fall of 1940, the United States having just begun the augmentation of its armed forces with its first ever peacetime draft, York was not only an active proponent of the measure, but offered himself up for active involvement as well. Although in his early ‘50s and well past service age, he served on his local county draft board. Recalling his own religions beliefs that had initially kept him from taking up arms, int was reported at that time that he was planning to speak with any local conscientious objectors in an effort to change their minds, similar to how his own had been in conversation with an officer.

As the war in Europe continued, he only became more vocal. In the Spring of ’41 at a banquet held at the Alvin C. York Institute, a private school he had founded, he beat the drum of war, strongly opining that the United States had a moral duty to be involved, and castigating the isolationists:

America now stands at a crossroads. This is no time to compromise with Hitlerism. Those who advocate such a source are following the illusion which has already plunged Europe over the brink of annihilation.

Late in July, he gave a speech to the Tennessean Society on the necessity of continued aid to the beleaguered UK, noting that "we must give England every aid we can, and as quickly as we can". American 'boots' on the ground were still something that he had reservations about, but he certainly had no doubt about which side was right, and America's obligation to be backing them.

These speeches, in fairly small settings, were nevertheless notable enough for the national news, and later in the year, York words echoing similar sentiments would be heard on a much more national stage in any case. Even aside from his Gary Cooper staring biopic that came out win the summer of ’41 - a strong image of the American citizen-soldier for the audiences’ minds, and likely to “be useful in awakening America in a time of crisis” as FDR himself noted - on the Armistice Day (Veterans Day) ceremony at Arlington Cemetery, less than a month before the United States would find itself at war, Yorks earlier remarks from a Memorial Day speech were utilized by the President himself, who quoted his response to critics:

The thing they forget is that liberty and freedom and democracy are so very precious that you do not fight to win them once and stop. Liberty and freedom and democracy are prizes awarded only to those peoples who fight to win them, and then keep fighting eternally to hold them.

Whatever we knew or thought we knew a few years or months ago, we know now that the danger of brutality, the danger of tyranny and slavery to freedom loving people can be real and terrible.

To be sure, none of this ought to be taken as the voice of American veterans. The result of that conflict had been considerably less than hoped for for many American veterans who had gone “over there” thinking they were making the world safe for Democracy, and felt little had come of it. Especially prior to 1939, York has earned pushback from groups like what would coalesce as the America First Committee, the ‘premier’ isolationist group in the country, although it by no means abated even after the invasion of Poland. In Memorial Day speech quoted by FDR, it was to those detractors that he had address himself, responding to the people who had asked him “You fought to make the world safe for democracy. What did it get you?”

Sources:

Mastriano, Douglas V. Alvin York: A New Biography of the Hero of the Argonne Lexington, Kentucky: University Press of Kentucky, 2014.

Lee, David D. Sergeant York: An American Hero Lexington, Kentucky: University Press of Kentucky, 1985.

Sergeant York may tell objectors how he felt. 1940. New York Times (1923-Current file), Oct 18, 1940.

HITS 'CALAMITY HOWLERS’: Alvin York Says Hitler 'Can, Will and Must Be Beaten' 1941. New York Times (1923-Current file), May 18, 1941.

PRESIDENT WARNS NATION IS FACING WORLD WAR AGAIN. New York Times (1923-Current file), Nov 12, 1941.

Sgt. york says america needs more planes, and submarines. 1939. New York Herald Tribune (1926-1962), Jul 14, 1939.

Roosevelt informs york film will help rouse U.S. 1941. New York Herald Tribune (1926-1962), Jul 31, 1941.

Tennesseans hear Sgt. York. 1941. New York Herald Tribune (1926-1962), Jul 04, 1941.

I'll take a stab at this as well, but more from the macro perspective.

I've seen this question come up before, and one thing I'd note is that it often seems to have an implicit assumption that World War I veterans were somehow a discreet group reacting to World War II. In truth, there was a vast amount of overlap: many World War II veterans were also World War I veterans. The time separating the Armistice of 1918 from the Invasion of Poland in 1939 was a little shy of 21 years, and so while few World War I veterans would have been in combat roles or the like as assigned to their younger peers in 1939-1945, quite a few were involved in the conflict, especially in senior political and military roles.

Just to run down a non-exhaustive list. For Americans who saw combat in World War I and played senior leadership roles in World War II, we can count Harry Truman, George Patton, and Douglas Macarthur (Dwight Eisenhower, Chester Nimitz and William Halsey also served during the war). For the British, Bernard Montgomery, Harold Alexander, Archibald Wavell, and Arthur Harris all saw combat in the First World War. Again, that's not an exhaustive list: heck, Winton Churchill (who spend a stretch in military service on the Western Front in 1915-1916) was a Cabinet member in the British government for both wars.

You will see a similar phenomenon among similarly-ranked political and military leaders in other European countries during the Second World War: Charles de Gaulle was a World War I combat veteran and ex-POW. Hitler and Mussolini both notably saw combat service in World War I, and were both wounded and decorated. Goering was actually famous for his World War I service prior to joining the Nazis, as he took over the "Flying Circus" after the death of "Red Baron" Manfred von Richthofen. A lot of the senior German officer corps served in World War I and saw action, including Erwin Rommel, Erich von Manstein and Heinz Guderian, just to cite some famous names. Indeed, in part because of the interwar restrictions on the German military, the German military in World War II was arguably disproportionately reliant on personnel who had pre-1918 service, especially for its reservists and junior and middle level officers. ETA - a very famous example of this would be Ernst Juenger, who was a highly-decorated World War I veteran who became famous after the war for his memoir, Storm of Steel, and who served in World War II as an army captain. Historian Jonathan House has stated that half a million World War I veterans served in the German field forces during Operation Barbarossa in 1941 (so something like a sixth of the German total). The average age of a German army or army group commander was 58 (or exactly from World War I veterans' cohorts), compared to 44 for their Soviet counterparts.

On the topic of the USSR, I think it might be of interest and perhaps a surprise that here too, senior military leaders often had World War I combat experience. Just to pick notably Marshals of the Soviet Union, this would include Ivan Konev, Georgy Zhukov, Konstantin Rokossovsky, Semyon Timoshenko and Semyon Budyonny. ETA - an estimate I'm reading via Brian D. Taylor is that by the end of the 1930s some ten percent of the Red Army officer corps were former Tsarist-era officers, including Soviet Marshal and Red Army Chief of Staff Boris Shaposhnikov. Tsarist era officers had been progressively replaced by younger officers in the 1920s and 1930s, in some cases through purges, such as of Mikhail Tukhachevsky, but quite a few were apparently still in service by the outbreak of World War II. Senior Soviet political leaders often had much less military experience from World War I, for the double reason that Bolsheviks were largely in external or internal exile during the war, and many "Old Bolsheviks" who had been major figures circa 1917 had been purged (imprisoned and executed) during the 1930s. Then again if we take a more expansive view, many still saw service in conflicts just after World War I. Stalin, for example, was in Siberian exile until 1917, and did not participate in the First World War proper, but was a military commander during the Russian Civil War starting in 1918. He spent that year defending a strategic town of Tsaritsyn on the Volga, that would become world-famous in 1942 under its new name, Stalingrad.

So: while there were plenty of World War I veterans who, like Alvin York, were on the home front during the war, there were a lot of World War I veterans who were direct participants in, and major figures and leaders of, World War II. This is one reason why it's so hard to describe a definitive reaction of veterans of the first war during the second.

From a German perspective, one of the best known WWI veterans, Ernst Jünger (of Storm of Steel fame), is a pretty solid source for how a veteran of the Great War viewed WWII and the Third Reich.

Jünger, despite being a hero for his near idiotic, death-defying bravery during WWI, had an odd relationship with the Nazis and later the Wehrmacht. On one hand, a lot of his later-deemed "conservative" writings were appealing to the Nazis, having written about societal decadence in Germany during the Interwar period. Yet, he never officially joined the NSDAP, and repeatedly refused to be used by the Nazis as a propaganda tool (to the point where he refused to be featured in any of Goebbels' radio messages). He also refused to serve in the Reichstag for the Party, when offered a seat. He also wrote a metaphorical novella that's now seen to be a critique of the Third Reich, On the Marble Cliffs. I personally view this work as a satire akin to Animal Farm. Basically, Jünger's book portrays a Utopian-like agrarian society that is then led to a dictatorship by a head forester because of that agrarian society's decadent collapse. A lot of historians believe the forester to be based on Hermann Göring. However, because of his status as a war hero, NSDAP/Far Right circles in Germany believed it to be an allusion to Stalin and the Bolshevik Revolution.

He served in France as an officer during the war, and wrote another book about his experiences, Gärten und Strassen. Basically, he saw the war and the Blitzkrieg as utterly chaotic, yet he in his own little world within the Wehrmacht seemed calm and orderly. It's pretty whimsical and goes into some philosophical discussions about the agency of men in conflict and their ability to impact the world around them.

By 1943, he, like a lot of Germans, began to see the war turning against the Third Reich and then became overtly opposed to the Nazis and the War thereafter. He wrote a book about his views on the topic, Der Friede, or in English "The Peace". This was his set of solutions for a unified Europe and what was to happen to Germany after the War. Between that and his association with multiple figures involved in the July 20 plot, he was removed from the Wehrmacht and kept a relatively low profile until after the War. Der Friede would be published internationally in 1947.

There is a LOT of information about Ernst Jünger, way too much for a single comment or post. But he's probably the best example of a high profile German WWI veteran that wrote about his experiences during the Third Reich and WWII.

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Sources:

Jünger , In Stahlgewittern (Storm of Steel), 1919

Jünger , Auf den Marmorklippe (On the Marble Cliffs), 1939

Jünger, Gärten und Straßen (Gardens and Streets), 1942

Jünger, Der Friede (The Peace), 1947

Vandenbroeck, "To howl with the wolves, or to fight them: Ernst Jünger in Kirchhorst (1939–1948)" Link

Hohendahl, "Reflections on War and Peace after 1940: Ernst Jünger and Carl Schmitt", 2008 Link