Extremal problems in ODE's and Pontryagin's maximum (minimum) principle. I think there should be many other examples from Soviet school, where mathematicians were sometimes forced to work on problems of "people's interest" (like optimal control in this case).

Extremal problems in ODE's and Pontryagin's maximum (minimum) principle. I think there should be many other examples from Soviet school, where mathematicians were sometimes forced to work on problems of "people's interest" (like optimal control in this case).