Pueblo West, Colorado — On the front door of the 20,000-square-foot marijuana dispensary here is a laminated sign warning every customer: “It is illegal to sell or transport marijuana to another state.”

“And you can guarantee people read it,” said Rick Hooper, general manager of the Spot 420 in this barren part of southern Colorado. “We make it very, very clear that this is the law here.”

Whether people obey is an entirely different question, and some neighboring states don’t think a warning sign is enough.

A border war has broken out between Colorado, where recreational pot is legal, and its neighbors, Nebraska and Oklahoma, where it is not.

In December, the attorneys general of Nebraska and Oklahoma filed a lawsuit to stop what they say is a steady flow of marijuana across the Colorado state line. Kansas is considering joining as well.

The suit, filed directly to the U.S. Supreme Court, seeks to strike down Colorado’s law legalizing recreational marijuana. It argues that Colorado’s statute conflicts with federal drug laws, which consider marijuana illegal, even in small amounts. [Read more at the Los Angeles Times]