In theory, when someone unearths anything over 300 years old, he is supposed to contact the Directorate-General of Antiquities, which has three months to examine the object or site before deciding whether to commandeer it.

ECONOMIST: A Phoenician statue for my coffee-table

So while Congress can't commandeer the states to do its bidding, it can offer them more or less any bargain it likes and states have the choice to voluntarily accept the terms or not.

WSJ: Review & Outlook: New Medicaid's Roach Motel