The main problem with this is that the U.S has never needed to field large military power, because massive oceans separate it from any other large economy. It's very hard to effectively project power against a continental nation from such a distance - and that logistical nightmare means that the U.S never really has a need for significant forces.
There are two ways of changing that.
The first would be creating a rival power on the American continent. With a 1898 POD, that is rather hard - you'd have to massively screw the U.S into a more destructive 2nd civil war of some sort, and either bring back the CSA or make a Mexico-wank. A somewhat plausible scenario would feature increasing rivalry with the British Empire, but that too creates many logistical problems for the British - Canada may take off a huge burden from supply lines as a base, but you would still need to bring troops in from all over the Empire. Exempting a spontaneous war, cooler minds would most probably prevail after a while and thaw relations. Though less-sane minds coming to power might be the POD. But it's still hard to get anything more than a powerful naval build-up.
Another way would be to have the U.S want to project its power abroad. Given how well isolationism served them, and how much sense it makes for a nation in their position, it's hard to see what would create important overseas U.S commitments that would need a powerful army. Colonial Ambitions in the Pacific and the Carribean are possible, but it's hard to get them to be anything more than limited naval skirmishes. After all, the U.S would face many of the same logistical problems fighting abroad that foreign powers would face fighting in the Americas - very long supply lines. That could, I supposes, be solved by gaining important allies abroad, and providing support in their conflict; but the only place that would be powerful enough to turn that into a massive military buildup would be Europe, and I have no idea why the hell the U.S would want to involve itself in one of the European wars for absolutely no benefit whatsoever.