The most obvious aspect I can think of is Uncle Sam:
...Uncle Sam didn't get a standard appearance until the well-known "recruitment" image of Uncle Sam was created by James Montgomery Flagg (inspired by a British recruitment poster showing Lord Kitchener in a similar pose). It was this image more than any other that set the appearance of Uncle Sam as the elderly man with white hair and a goatee wearing a white top hat with white stars on a blue band, and red and white striped trousers.
The image of Uncle Sam was shown publicly for the first time, according to some, in a picture by Flagg on the cover of the magazine Leslie's Weekly, on July 6, 1916, with the caption "What Are You Doing for Preparedness?" More than four million copies of this image were printed between 1917 and 1918.
It's worth noting that Woodrow Wilson was re-elected in 1916 in part because of a pledge to continue to keep the US out of the Great War, and it was the combined events of the Zimmerman Letter (a telegram sent from Germany to try and incite Mexico to go to war against the US; this was intercepted by the UK and sat on until the timing was right) and the sinking of the Lusitania (a passenger ship carrying American citizens which was sunk by a German U-Boat, although in fairness I believe that it was since found to be carrying munitions) that caused him to change his mind or at least marshal the requisite outrage needed to go back on his previously stated beliefs in public.
As for peer pressure, there was a huge amount of it, a lot more than you might think. The derogatory statement was "slacker", used to denote anyone who was not either going overseas or helping with the war effort. Even not helping enough was considered slackerish behavior. If memory serves, future boxing heavyweight champion Jack Dempsey got a lot of guff when a promotional photograph of him pounding on something for the war effort (in a factory, not a boxing ring) showed him wearing expensive shoes.
I have no idea how you'd even begin to measure what percentage of the country enlisted due to peer pressure vs. other factors. That would be tough to measure in today's society as it is; in World War I era America, I'd think it's downright impossible.