What federal law? In 1919, the 18th amendment was ratified. This granted Congress the authority to pass national legislation to prohibit drinking alcohol. Without the amendment, the ensuing Volstead Act would have been unconstitutional. The federal government had exactly the same degree of supremacy in this area of the law in 1918 that they do today (9th amendment, 10th amendment, 14th amendment, supremacy clause, commerce clause). And they needed the 18th amendment to prohibit alcohol.
The 21st amendment returned the authority to decide whether an intoxicant was legal or not to the states, as it had been prior to the 18th. The 21st did so by wholly repealing the 18th. The 21st did however affirm that moving a legal product in one state to a state where it was illegal would be illegal, and since such a thing is by definition interstate commerce, the federal government has authority there.
The Controlled Substances Act (and other anti-drug legislation at the federal level) existed in a constitutional gray area so long as all states prohibited those substances as well. As soon as any state legalizes any substance prohibited by the CSA, that gray area evaporates leaving only black and white behind. The federal government simply lacks the authority to criminalize marijuana against the wishes of a state within the borders of that state.
Title 18, Chapter 13, Sections 241 and 242 of the U.S. Code make it a federal crime for a public official (a police officer or DEA agent qualifies as such) to use their official authority to deny a citizen their rights. Even if federal agents are not bound by state laws, they are absolutely bound by federal laws. While the idea of a lone federal agent trying to serve an invalid federal warrant while unarmed is amusing, it's unlikely to happen. Which means that just about any attempt by the feds to unconstitutionally enforce the CSA in Washington would result in a conspiracy against rights under color of law with threat of dangerous weapons. Under 18USC241, that's punishable by execution or life in prison.
Granted, getting any police agency to police itself against its own best interests is probably futile. But it shines a wholly different legal light on shooting at federal agents during a marijuana raid, eh?