Force Modernization is a Military Policy Card in Civilization VI: Gathering Storm. It was added to the game in the Antarctic Late Summer Update, in which it inherited the effects of Professional Army.
Force Modernization halves the Gold and strategic resource costs of upgrading units, allowing you to modernize your military more quickly. Matthias Corvinus can use this policy to reduce the cost of upgrading units levied from city-states to a pittance.
Don’t think of it as an arms race! Think of it as a way to keep the military-industrial complex fed and happy instead.