It depends on how far back you go and who you contact about he incident and the evidence to solidify the claim.
9/11 could have been stopped relatively easily with a few days notice to national security. The air force could have shot both planes out of the sky. Just call the civilian deaths a tragic casalty of terrorism and use it to help fuel the war. The twin towers could have been shut down that day.
But you've gotta materialize right in front of the commander in chief, bring a mountain of carbon datable evidence like news papers and original classified docs, and hope that you don't get brained on the spot before you make your case.
As for covid, you probably can't stop it but maybe better warn and prepare world governments so they can get their population ready through subtle conditioning like trying to get wearing mask be a fashon trend or advertising bidets heavily as the new rich yuppie thing to show as a status symbol.
Scientists have been talking about a big pandemic for years before Covid. Nobody cared. If I ever get to time travel, I'll use it for personal gain. That's the only way something good'll come out of it.
Bringing evidence from the future is cheating. They wouldn't believe you if you told them the truth but if you say you are a terrorist and placed a bomb in the world trade center, they might evacuate it
Cheating? I didn't think there were rules covering time-based warfare. Damn. Wheres the fun if you can't cause some temporal paradoxes and get a few people stuck in Groundhog Day scenarios. Where's the freedom?
You dont notify the government directly, you notify the news desks of MSNBC, Fox, ABC and the major newspapers. Do it via express postage envelopes a few days out, include any publicly available info on hijackers, flight numbers and times.
The Government might ignore it as a hoax, but not if they know that the information has come via every major news outlet in the country. They have to act.
But what could you do without throwing your life away? Basically you would have to hijack a plane to increase security. Regular appeals by a person claiming to be a time traveler... might not be taken seriously.
Hijacking a plane wouldn't be enough. Plane highjackings were nothing new. They were even common back in the 60's. Conventional wisdom was you cooperate with the highjackers. They have to land eventually.
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If you're a time traveller you could build up credibility with other things too. Start contacting a journalist with predictions of future headlines. Go back to October 2000 and say what's going to happen with Bush v. Gore. Then in like July 2001 meet up with them, explain that you're a time traveller which is how you know this, and show them the video.
Go to the cops and prove you are from the future. Get there a few days early with the lottery numbers on a piece of paper or smth. That might get them to take you seriously. Of course they will never let you go back.
Just call with a bomb threat, they take that shit seriously and it would have emptied planes and buildings. Call the PD and threaten both towers, the pentagon, and the airport.
They wouldn't buy the attack on the Pentagon. I can't see a scenario where you could threaten the military central of the US that somehow it was endangered from within or outside.