Well I can tell you something about the Great War. If we (The United States of America) had entered the war on the side of the German Empire the Atlantic would just become a busier mess of ships, navies and sorties. American ships would engage the Royal Navy in the Atlantic and the Pacific, American infantry would invade Canada.
If the Royal Navy can be mopped out, or at least constricted to the British Isles then the US Navy can starve out the Islands. Once we (the USA) have done that we can prepare an invasion of France!
The White Army would win in Russia with German support (Monarchy over Bolshevism). Pretty much a more conservative world without Nazis, cupcakes all around!
You do realize that the USN was a horribly unbalanced fleet in the WW I era, right?
How about that the Germans CAUSED the Red Revolution as we know it IOTL? The Kaiser had his cousin's blood all over him to his dying day.
The U.S. has good, bordering on spectacular, battleships, arguably operating the best in the world with the
New Mexico class in 1917 (the U.S. MK 6 14" gun had better range, armor penetration and rate of fire than the RN 15" MK I) but Congress had failed to allow for sufficient sufficient light forces to support the battle line. The USN had ZERO BC, while the British fielded 9, overall the RN out numbered the USN 2-1 in capital ships (with the difference in pre-dread being 3-1 in the RN's favor, while the UK had a 2-1 advantage in Armored Cruisers). The United States operated THREE light cruisers while the RN had roughly 45 and around 80 destroyers to the British 240+. This is a force insufficient to provide scouting and escort for the excellent U.S. Battle Squadron, something that would have allowed the RN to defeat the USN in detail. The USN also lacked coaling stations anywhere near the Continent.
As far as U.S. infantry invading Canada, I am compelled to ask: WHAT U.S. infantry? The American Army was virtually a non entity in 1914 while Canadian troops were the elite of the BEF, arguably THE elite force of the entire War, with the Best General officer the Entente produced in the entire War (Sir Arthur Currie). Eventually the U.S. would have prevailed in North America, but it would have taken YEARS to do so.
Where the U.S. WOULD have done enormous damage to the Entente would be in the material and economic arena. The U.S. produced over 1,000,000 Enfield "P" rifles for the UK, as well as huge amounts of other weapons, uniforms, munitions, food stuff and, most importantly, ready credit. In 1917 alone the U.S. trade to the Entente was 3.2 BILLION dollars, of that $2.7B was on credit.
The U.S. would not have had to defeat the Entente in the field (a good thing because it would have been a very tough row to hoe) if it had wanted to assist the Central powers. All it would have had to do was make everything cash and carry. The Entente lacked the cash and would not have been able to carry anything back without it.