It will be a long process, but I suspect that the active-duty Army will become a cadre force similar to that of the 1930’s. This is the historical role filled by the US regulars.
The Marine Corps? I suspect that they will survive, but maybe not in the form of a multi-division force.
The Navy will fight to maintain a large force structure and their main competitor for budgetary funding will be the USAF. I expect that large-deck aircraft carriers will go the way of the dinosaur over the next 20 years as the Navy looks to do more with its LHA/LHD platforms and unmanned drones.
It’s all about $$$’s and mission. There will be sudden sharp shifts, like plate tectonics.
Wrong model, wrong history. The Army was cut to nothing by cheeseparing, tax-cut-loving Harding and Coolidge Republicans (the Dems can plead out innocent on that one). The Navy was barely kept afloat, the Marine Corps was tiny, and US troops and marines trained with wooden guns and sacks of flour dropped from airplanes.
That's why we lost the Philippines, even after almost two years of bulking-up and calling up the Reserves and National Guard outfits.
We don't want to go back there. Especially since we know, ever since World War II, that every unpleasantry we've faced has been a "come-as-you-are war" with the exception of Vietnam. FDR was the last President to enjoy the luxury of time to prepare even halfway for a really big war.
The Armed Forces are already drastically drawn down from Gulf War levels, thanks to cutting by both Bushes, Clinton, and now Obozo. This time, it's LBJ's vote-buying programs that will have to eat it, and that means throwing the 'Rats out of office and out of power, and going in there and doing what is necessary. Only this time, we can't let McQueeg stab us in the back like he did Bush 43, when Bush tried to reform and de-politicize Social Security.