I don't have an exact date, but I was thinking about American decline in my lifetime earlier this week, and thought up a short list of things that have contributed to it in the last ~30+ years. This isn't extensive or anything, but it's some of the things I think have contributed the most.

• Collapse of the USSR, shock therapy neoliberalism in Russia and interfering with the 1996 elections (means Putin is put in charge, since he was Yeltsin's hand picked successor)

• Clinton's presidency and NAFTA, leading to even further deindustralization than during Carter's and Reagan's presidencies

• NATO bombing of Serbia

• Repealing the Glass-Steagall Act, most people don't realize the significance of this, but it directly led to the 2008 crisis. IMO, if you really want to point to a year when American decline started, it'd be 1999 when this happened, the ultimate act of 90s neoliberal hubris

• Dot-com bubble bursting, which is another overlooked event IMO, but another sign that neoliberal capitalism and the notion that the market can regulate itself was a fairy tale

• Election of George W. Bush, which was of course a farce, essentially a coup with disrupted recounts in Florida (Brooks Brothers riot) that led to the Supreme Court handing the presidency to Bush, nominally to avoid hurting his feefees

• 9/11 and the immediate aftermath, rounding up Muslims, passing the Patriot Act among the sus anthrax letters and whatnot. The US had immense amounts of global goodwill, which they proceeded to quickly squander. 2001, another date you could pin American decline to and you'd get no objection from me. Note to Israeli leaders post October 7th: <- you are here

• The invasions of Afghanistan and Iraq, the first of which was excused due to 9/11 and the notion that Bin Laden had to be caught (the Taliban were in fact willing to hand over Bin Laden as long as it was to a neutral country, so there was no need to start a war). The Iraq War was such an unnecessary thing that even the US' NATO allies like France and Germany were not willing to participate in it, and IMO 2003 would be another good date you could point at as the start of the decline

• The 2008 crisis, which was 100% caused by the US finance sector (see the repeal of the Glass-Steagall act above). Not only that, but newly elected president Obama said he'd bail out the banks and any other company he thought were "too big to fail", nevermind the fact that the fucking investment banks caused this whole crisis and now they'd be getting away with everything

• The NATO bombing of Libya, which came after Gaddafi handed over his nukes and expected to be made a member of the rules based international order. Not only did this led to Hilldawg's "We came, we saw, he died" thing, but Putin reportedly viewed the footage of Gaddafi's death with disgust, which directly leads us to...

• The Ukraine coup with the help of neo-Nazis. Of course this can be directly traced to the current war in Ukraine, since Russia considered Ukraine joining NATO unacceptable. They didn't like the Baltics and the like joining, but it was always clear that Ukraine becoming a member was just that one step too far. The post-coup government amended the constitution in 2019 to include joining NATO, and here we are now...

Yeah, this is already a super long post and doesn't include things like Syria which I honestly don't know that much about (hopefully someone else will fill in those gaps), but I'd say the start of the decline was either 1999 or 2001.