Many Americans may laugh off Trump’s latest outrage, but Trump crossed an important line. It is one thing to float a cockamamie idea that no one believes is serious or will go anywhere. “Let’s buy Greenland!” Yes, very funny. A good distraction from the economy, the failure to deal with white supremacy, White House staff problems, or whatever is the news of the day. It is quite another to use leverage and impose costs on Denmark in pursuit of that goal—and make no mistake, canceling a presidential visit is using leverage and imposing costs. What’s next, refusing to exempt Denmark from various tariffs because it won’t discuss Greenland? Musing on Twitter that America’s defense commitments to Denmark are conditional on the negotiation? Intellectual justifications from Trump-friendly publications, citing previous purchase proposals and noting Greenland’s strategic value and abundance of natural resources? (That last one has already happened.)

This is the kind of thing the Russians and the Chinese do. It is territorial revisionism—the use of national power to acquire territory against the desire of its sovereign government and its people. The use of leverage would also call into question the U.S. commitment to the 1975 Helsinki Final Act, which is the cornerstone of stability in Europe. In it, all parties, including the United States, commit to “refrain from any demand for, or act of, seizure and usurpation of part or all of the territory” of all states in Europe.