Marijuana laws: When weed is legal in Canada, Americans are …

The Ambassador Bridge, which connects Windsor, Canada, and Detroit. In October, recreational marijuana will become legal in Canada, presenting a challenge for U.S. officials at the border, where possession of the drug violates federal law. (Brittany Greeson/The Washington Post)

Recreational marijuana has been legal here in Washington state since 2014. Adults just a few miles away in Canada also will be able to legally buy and smoke marijuana for pleasure starting in October.

But between them stands the U.S. border, a more at