Since when?
Almost every war has been based on lies and propaganda.
Johnson, a true believer in the Domino Theory and responsible for the Gulf of Tonkin hoax, didn't need concrete evidence for other conflicts.
Hence the US government's justification for obstructing or burying investigations into the conspiratorial leads in the JFK case.
It's generally a good practice to avoid asking the questions that you don't want to know the answer to...
Sorry, there was no Gulf of Tonkin hoax. Hoax as in deliberate deception. Are you talking about the second attack? No, there was no second attack by the North Vietnamese; that was based on faulty information. But it wasn't a hoax. It was the fog of war. And there was a first attack. And LBJ had to rely on information given to him by his people. He didn't make this up.
In my view, even if there was no attack at all at that time the US and Hanoi were headed for a direct confrontation. At some point, at some time. The North wanted to take over the South and after Diem's removal they stepped up their attacks. The US decided that that was a threat to our security (it wasn't but that's how they - JFK and then LBJ - saw it). It was inevitable that the two sides were going to clash given the nature of the situation. Unless we were to simply leave.
As to the assassination: We had the WC, the HSCA and numerous investigations by the news media. CBS news, ABC news, the Washington Post, the NY Times.
You believe all of these investigations were obstructed by the government?
Look, I know I'm defending in LBJ a horrible guy. Clearly, he was corrupt, he lied and he personally was in some ways worse than Trump. I said some <g>. But I think he was sincere in going after racism, in dismantling segregation, and in trying to help poor people. And I think he truly hated the war. He didn't want it but felt trapped. That doesn't excuse the duplicity, the incompetency; but it does go to his motives which I don't think were pure evil.