They wanted to weaken "subversive" elements within the nation. Remember this was the Cold War era - they were terrified of the youth getting ideas into their heads that were "un-American". McCarthyist BS. Get the kids dosed up, villify them in the press and take out their real enemies (perceived), such as Civil Rights leaders and more radical groups such as the Black Panther Party. Make the hippies the scapegoat and murder Fred Hampton who was preaching equality and feeding children. Manson was likely MK Ultra and look at how they used that horror to kill the movement and usher in a new age of conservatism, leading up to Reagan who funded the Contras with the help of Old Man Bush, creating the crack epidemic. Years later, Bush Jr. gets in and takes the empire and its satellite States into Afghanistan, where all of the heroin in my part of the world happened to come from. Before that, Vietnam, which funnily enough was conveniently placed to bring heroin into the states. All the while, the CIA were testing out chemicals on people as the FBI were going after commies/perceived commies/radicals etc whilst giving various mobs the green light to bring drugs into the country. There are many layers to this and the more you look into the hippie movement and all that, the more you realise it was all part of a big Psyop or at least a collection of interrelated psyops, with MK Ultra being just one that we know of. And look at how many of these "hippies" ended up becoming yuppies? Most of them were just following fashion. Frank Zappa had some interesting things to say about this also. He didn't touch drugs and has even been accused of being a CIA asset of sorts. Go figure. Also, Whitey Bulger claimed to have been fcked with in a manner akin to MK Ultra and he was a protected high level FBI informant for decades. And as I said, the Manson family stuff, other weirdo serial killer types allegedly, all designed to strengthen the state in some way.
All of this is a bitter pill to swallow as many of us love our drugs and have fond feelings when it comes to these movements but think about the bigger picture. Think about what's going on now with fentanyl, tranq and the fact that none of this could happen without st least some complicity via corrupt government agencies. The "war on drugs" became a cover for the war on movements of national liberation (and communism), mostly in Latin America. Now, it is hard to understand why they are letting it happen but I believe that there are two fsctors: 1) keeping key sections of the population marginalised and essentially neutralised, i.e. poor people who might start organising poor the Panthers did (an FBI informant got Dr. Huey Newton into cocaine, just an example that came to mind) and 2) money, the most obvious one. Billions and billions of dollars, flowing through the intricate global financial network. Look at the Brits and the Opium Wars on China. None of this would happen if it wasn't making money, whether for the upper classes and the shadier parts of their portfolios or for the prison industrial complex, the cops, the DEA, the "intelligence" agencies - all of whom combined are seemingly unable to stop the flow of drugs which actually gets bigger and bigger every year. Money, social control and justification for marginalisation/heavy policing/privately run prisons which make enormous profits from slave labour and of course helping to fund insurgencies or counter insurgencies, depending on which side of America's "interests" a group is fighting in the benefit of.
One last thing, Hunter S. Thompson accused Timothy Leary of being a rat (could've been Ken Kesey - forgive me if I'm wrong) and if you read his stuff beyond Fear and Loathing, you'll realise that he was very close to government sources himself and would likely know such a thing. I think people get too hung up on the "truth serum" stuff which of course was a part of it but the picture was much bigger than that, and the CIA etc. weren't as naive as some people here have made out (I. e. the classification of drugs etc.)-i think that some very sinister, very clever people realised that drugs could be used for far more World changing things than getting the truth out of some Soviet spook. One could almost conclude that elements of how drugs have been dealt with by government agencies could be seen as examples of chemical warfare.
Edit: forgot to mention a key element that concerned the US government greatly, or at least the shady alphabet agencies, and that was the anti-war movement. Turn the anti war hippies into the enemy and bring the masses back to a conservative mindset, helped much by atrocities such as the Mason Family horrors (read Tom O'Niell's book on them, and Ed Sanders' book 'The Family'). But yeah, look at what happened to San Fran after the summer of love - kinda like Kensington, Philly now no? Job done. Time to vote in Nixon.