Keep It In-House.
- States can't meet with, exchange gifts with, or look the wrong way at the leaders of other countries, especially "Kings" and "Princes" (6.1)
- Two or more states can't enter into a special alliance with each other, sans permission from Congress.
- States can't make laws or collect tariffs that violate treaties between the U.S. and other countries.
- States can't have their own standing armies or standing (floating?) navies. But they do have to keep up local militias, just in case things get ugly.
- States can't start wars, but they can protect themselves if they are invaded. They're also free to kill pirates. Jack Sparrow was a big problem back then.