I imagine there are other threads about this, and please feel free to point me in that direction.
I'm currently helping to create the framing of a wargame which imagines an alternate 1919 where the US entered World War I on the side of the Central Powers in August 1914. The actual point of divergence for the wargame is much earlier, but at the risk of making the scenario implausible, I want to focus on the military aspects not the political just so that this thread is more focused.
I'm familiar with the state of land warfare in 1914, but pretty unfamiliar with the disposition of the world's navies at that time, beyond the folk history of dreadnoughts and u-boats, and I imagine the main thing that would change with the US as a Central Power in 1914 is the Naval balance of power.
In terms of land warfare, the US probably would have quickly invaded Canada, whose major cities are largely close to the US border. The Canadians did not deploy their Active Militia in World War I, opting instead to outfit a Canadian Expeditionary Force separate from the PAM. That would change in this timeline.
Interwar Royal Navy contingency plans did not place a high priority on defending Canada in the event of a war with the US, whereas interwar US plans placed a high priority on taking Canada (assuming wrongly that Canada would be the main front of the war, and that the British would use it to invade the US.)