Louisiana would either remain part of the Spanish Empire, be given back to France again, or conquered sometime by Britain (probably during the Napoleonic Wars). If it remains part of the Spanish Empire, it would most likely gain independence along with the rest of the New World, and the Allies could trade with it during TTL's WW1; if it's ruled by France or Britain, obviously the Allies also benefit from its resources, in a more direct way than IOTL. Ditto Texas. The Mid-West probably gets annexed by Britain and joined to Canada, so it will also be directly helping the war effort from the beginning. Alaska is likely ruled by either Russia itself or Britain, so any resources it brings to the table (oil, mostly) can likewise be used. Hawaii probably gets occupied by Britain, but even if it doesn't, it's too small and out-of-the-way to have much of an impact on the course of the war. About the only really valuable part of the Mexican Cession was California, which the alt-Allies will be able to trade with, that is assuming Britain hasn't already occupied it anyway.
So, in short, if the "US" doesn't expand but remains confined to the East Coast, the land they occupied isn't going to disappear; it will either be occupied by other colonial powers, most likely ones (Britain, France, Russia) which are involved in WW1 anyway, or it will consist of independent Western countries, which the Allies can trade with. That's one of the ways in which the Royal Navy blockade helped the Allies win, and would help them ITTL -- it allowed them to get resources from the whole extra-European world, whereas the Central Powers were limited to what they could produce themselves.
None of that seems to have held Europe back to any noticeable degree. Remember that the balkanised US states would only be small compared to OTL's USA; by international standards, they'd mostly be normal-sized countries.