I think going to war with the US over food is going to be an absolute last resort. In a major war, the participating nations are all going to be diverting food production into sustaining the military, including the production of biofuels. They will do this to the extent that they will short their own civilian population. The US did this during WWII and I have no doubt that we would do it again if the situation arose.
I do believe that those nations whom we supply with foodstuffs would take a run at us, and any other supplier nation to them, through economic and diplomatic means. That is, China would tie its exports to the US with our exports of food to them. We don't ship food; they don't ship computers, clothing, etc. I think the same would happen with oil producing nations who export to us with much more immediately damning consequences. Pushed far enough, I think even our Government would loosen restrictions on domestic drilling and other fuel production enterprises, but it would take time to ramp up production. I am less than certain we have that much time to spare with our current situation.