What would the effects of both wars be if (for whatever reason you can think of) the US were to join the Allies right off the bat at the start of both wars?
Would depend onto the circumstances as to why, which would affect how heavily America gets involved. [For instance there would be a lot of difference between a US in WWII because of Pearl Harbour and a US which had seen a US dow pushed after a relatively minor clash between US and German units in the Atlantic say]. However would have a big impact, especially in WWII.
In WWI, presuming that US commitment is substantial, it could have a sizeable force committed say in spring 1916, alongside the big British push on the Somme say. Would very likely suffer heavily because of lack of experience but would learn and the Germans would be under even more pressure. Likely that once the US was able to commit large forces, by about that date, that barring some major problem for the allies the central powers would realise the game was up and make peace. [At least Austria once Karl came to the throne].
In WWII, presuming like OTL the US had largely demolished after WWI, it would only be able to commit relatively small forces early on. However the Germans were working on such a narrow margin that it could make a significant difference in the battle in the Low Countries. Furthermore US commitment to the allies would be a big moral boost and even if France fell, the French government might well be encouraged to fight on from N Africa. Also the inclusion of the USN would be a big boost to the allies, cementing their control of the seas.
Where this might backfire would be that since the Anglo-American powers would be such a substantial bloc you might see Hitler persuaded not to attack Russia. The Germans wouldn't really have the capacity to do this given the extra forces they would need elsewhere and they couldn't afford to antagonise their primarily supplier of many vital raw materials. True Germany couldn't really pay for this but Stalin might well decide to continue support to enable Germany and the western powers to weaken each other in a long war of attrition. This could be very costly for both sides.
Given US active involvement, even if France falls, Mussolini may decide not to get involved as the war would manifestly not be over.
Steve