As far as I know, possession and trade of marijuana is illegal in the United States according to federal law. However, the state of Colorado (as well as some others) recently legalized marijuana for recreational use.
Title 21 United States Code (USC) Controlled Substances Act classifies cannabis as a controlled substance. How is it possible for states to legalize a substance which is illegal according to federal law? Couldn't the FBI and DEA enforce federal law and arrest marijuana traders and consumers in Colorado and Washington?