In 1914, there was a very large part of the US population that had German ancestry. German was a very common language taught in schools, as in some fields ( like music, theology and philosophy ) Germans were extremely important. So, there was a good bit of popular sympathy available for the German point of view at the outbreak of WWI.

That quickly began to diminish with reports of American journalists covering the invasion of Belgium, as well as the reports coming from the US embassy there, as the US legation tried to mitigate and mediate to minimize Belgian casualties. There was also the fact that telegrams and wire dispatches from Europe came through a cable laid between England and the US, and England could therefore control a lot of the information coming from the War. The English also mounted a very effective propaganda campaign in the US, wisely avoiding arguing for real US intervention in the conflict but pointing at Germany as the aggressor, and keeping open the flow of US material assistance to Britain. The German propaganda effort was not nearly as good.

One important journalist who worked rather hard to get the German point of view was H. L. Mencken. He was biased: he came from a family that was part of the very strong German-American community in Baltimore , knew and liked the language and culture, and especially admired Nietzsche. He was also an isolationist, believed in small-government. He praised the German cause at the outbreak in 1914, saying the War would bring more of the superior German culture to the world. He travelled to Germany as a correspondent of the Baltimore Sun at the very end of 1916, and was warmly received by the German government, especially when he expressed the hope that he could do more justice to the German side of the conflict, and was soon writing in their support. However, after only a month, at the end of January 1917 the German government announced that unrestricted submarine warfare would be used against all shipping in and out of Britain, and all US citizens were asked to leave- and even the Germanophile Mencken was forced to leave.

On his return to the US, Mencken would continue to write pro-German columns for the paper. But anti-German hysteria really began to build in 1918, with the entry of the US into the war. German names of streets and cities changed, and there was even the famous lynching of Robert Praeger in Missouri. Mencken was soon silenced, his column taken away from him for the duration of the conflict. But he would still maintain, even years afterwards, that the US had backed the wrong side. On Armistice Day , 1931, he wrote in the Sun,

The United States made a similar mistake in 1917. Our real interests at the time were on the side of the Germans, whose general attitude of mind is far more American than that of any other people. If we had gone in on their side, England would be moribund today, and the dreadful job of pulling her down, which will now take us forty or filthy years, would be over. We'd have a free hand in the Pacific, and Germany would be running the whole [European] Continent like a house of correction. In return for our connivance there she'd be glad to give us whatever we wanted elsewhere. There would be no Bolshevism in Russia and no Fascism in Italy. Our debtors would all be able to pay us. The Japs would be docile, and we'd be reorganizing Canada and probably also Australia. But we succumbed to a college professor [President Woodrow Wilson, whom he nicknamed "the Archangel Woodrow"] who read Matthew Arnold, just as the English succumbed to a gay old dog who couldn't bear to think of Prussian MP's shutting down the Paris night-clubs.

Panczenko, O. (2016). Some Notes On Mencken in the First World War. Menckeniana, 215, 7–15. http://www.jstor.org/stable/26485909