It reached its peak halfway through Jimmy Let's-Federally-Decriminalize-Marijuana Carter's term, yes.
Yep, if you're going to claim "getting serious" caused the decline, then Jmmy Carter gets credit for the onset of the 10 year decline.
And it continued to fall until the events at Waco, Texas. What does Waco, Texas have to do with the low point in drug use? Absolutely nothing, the same as elevating the WOD to cabinet level.
The WOD was elevated to cabinet level status to better fight the WOD. If demand increases after falling for 10 years and supply explodes, it's a failure.
Let's look at a chart of marijuana "Past Month" use so you know exactly where I'm coming from:
First of all, marijuana use fell 65% in the period from 1979 to 1993. It then remained relatively flat for 8 years. It is up slightly (NOT explosively) in the last few years.
Why is it up? My guess is that the increased calls for medical marijuana and decriminalization have a lot to do with it, just as it did in Jimmy Carter's days.