I don't think you understood what I meant by "buy options".
So let's flesh that out a little. Suppose steel costs $2/unit from a US firm, but costs $1/unit on the world market. We can go to companies in various other countries and pay some nominal fee (say, $50k/year) to buy an OPTION to buy their future steel output, not at the current price, but at twice the world price. So in the best case scenario there's no war, the steel company gets paid a nominal fee for an annual option that never gets exercised, and the US military saves far more by buying low cost steel through the years than the cost of the options. But if there ever is a war, we execute the option and get to buy (on 6 month's notice) that factory's full capacity at a 100% markup, crowding out whoever else they would otherwise have sold that steel to at a much lower price. So in wartime we pay foreign companies exactly what we would have had to pay a US firm - twice the world market rate - without first having to wait to build any US factories!
The foreign firm just has to allow for this in their contracts - that they reserve the right to cut off any buyer (paying a suitable penalty fee) with 6 month's notice. Now our stored 6-month supply is exactly enough to tide us over until options can reasonably be exercised (and penalty fees paid) that provide us with substitute sources.
We sign those sort of contracts with firms in a variety of countries in a variety of geographic locations. Unless our primary low-cost supplier gets mad at us and ALL those "safety" countries also get mad at us at the same time, we have an adequate safety margin.
All our steel - heck, all our ships - gets made for HALF the current cost until such time as there actually IS a war with our main supplier, whereupon it all gets made for exactly the price it would have cost to make domestically, had we kept that industry around.