Every country subsidies their agriculture for national security purpose. You don't want an enemy to starve you in case of a big war.
Even if one grants this, it does not make the case that any particular set of subsidies is justified by that reasoning.
Name one country which is fully self-sustaining.
Agricultural subsidies exist primarily due to lobbying, the "we don't want to starve" argument is nothing more than an excuse used to justify the fortunes handed out to corporate-scale farmers.