The US and China are both having worsening pension and medical costs which will vastly restrict the money available for military programs and wars.
By 2040, China could have $10 to 100 trillion in unfunded pension costs. China is about to experience the most rapid aging crisis in human history, with the ratio of workers-to-retirees shrinking from 8-to-1 today to 2-to-1 by 2040.
China’s military devotes more than 1 million troops (roughly 45 percent of the active-duty force) to internal security and border defense. The cost of maintaining these units drains at least 35 percent of China’s military budget, a chronic “domestic drag” that puts robust power-projection forces further out of reach for the PLA.
The US devotes ten thousand + troops and more mercenaries for middle east wars. The troops get turned over during the multiple decade perpetual wars and the war veterans rack up far higher medical costs. There is also the higher US war on terrorism and homeland security costs. These costs appear likely to continue for decades.
2013 projections, from the Center for Strategic and Budgetary Assessments, show that given historical trends of cost growth in personnel spending and O&M, those two items will consume 86 percent of the allowed DoD budget by 2021.
There will still probably be small wars but bigger, tougher and more expensive adventurism would seem likely to be curtailed. Older and broker populations probably would not step up to big wars.
Russia finances currently suck but they still have the Ukrainian war and fought in Syria.