The Heritage Foundation claims that farm subsidies are not for national security.[1] From this we can conclude that the purpose of farm subsidies is almost certainly national security. Which is a good thing, because farm subsidies fail to prevent hunger as you claim.[2] Om the other hand, it seems to be less for security than as "socialism for the rich"[3] given how the money is funneled.[4]

[1] https://www.heritage.org/taxes/commentary/farm-subsidies-vs-...

[2] https://en.wikipedia.org/wiki/Hunger_in_the_United_States

[3] https://en.wikipedia.org/wiki/Socialism_for_the_rich_and_cap...

[4] https://www.npr.org/sections/thesalt/2019/12/31/790261705/fa...