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.