My best (stupid) guess is weapons. US gov has a ton of weapons. Already sells tons of weapons. Now the prices will always remain stable. Other countries would love to know that 30 million usd would always be able to buy an f15. Other countries declaring war will increase the value of the USD, as buying weapons from us government will decrease amount of money in circulation.
Well to get the thing that is backing the dollar, they would have to turn in us dollars that would be taken out of circulation . So a bunch of countries at the same time would get a bunch of USD and turn them in.