What's the delta between the environmental impact of growing wheat and beef etc locally vs shipping it 8000 miles?

Trans-oceanic shipping and trains are surprisingly low-impact per unit of food. When you add in the environmental impact of things like growing rice in Texas deserts and almonds in California, the results can easily come out in favor of trade.

There are other considerations, too. Favoring local products is a good way to keep poor food-exporting countries poor. I don't want that, though I understand some people have different priorities from me.

This is great until a major conflict happens between the US and the country responsible for a major food staple, and domestic production is decimated by imports, and there’s a food shortage.

That sounds like excellent incentive for everyone to try to avoid and reduce the severity of conflicts.