Federal law must insure domestic Tranquility, Justice, Defense, and general welfare between all states and it's people.
If the American people in this example "Washington state" have passed a law (legal marijuana) that's in conflict to Federal law. It is the Federal governments job to uphold Justice and not come after the people who voted to pass said law.
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.