Randall Parker has a fun post on hypothetical* time machine travel and what he fantasizes about being able to accomplish if given access to the requisite technology. In that spirit, the first three things that came to this rank amateur's mind:
- 326, modern-day Serbia. Locate the place Constantine's (probably) illegitimate but competent son Crispus was being held and hold off the assassin the irascible emperor dispatched to off him. Next, make Constantine and his court aware that Fausta framed Crispus and allow the emperor to gas her as he did.
The intent of this bit of social engineering being to avoid the in-fighting between Constantine's three succeeding sons and prevent the breakup of the empire and the eventual collapse of its western half. A contiguous empire is maintained through the time of Justinian (or whoever wore the purple as a consequence of my meddling) and the wealth secured by Anastasius isn't squandered on a quixotic and crippling quest to make the thing whole again.
Parenthetically, in a sort of wish-for-more-wishes move, I'd go back past Seneca all the way to Aristotle to inform tutors that their most important function was to instill a sense of just how damned important succession planning is. It should be the first order of business! In WEIRD societies we take the smooth transfer of power for granted in our own countries, but that's not even the contemporary global norm, and it certainly hasn't been the historical one. Staying with Rome, it barely made it 60 years after Augustus before a lack of prudent succession planning threatened imperial collapse.
- Circa 1190, England. Prince John would be informed in no uncertain terms that he was not to repeat the treachery he'd taken part, in league with his older brother Richard, against his father by doing the same to said older brother while the latter was on crusade. So doing would result in him becoming a unwilling teetotaler. Not because there wouldn't be anymore alcohol in England, but because he'd be dead (a threat I'd rather not have to make good on, because as a good little-R republican, I wouldn't want to indirectly keep the Magna Carta from coming into existence!). Instead, he would loyally and dutifully work to maintain his brother's holdings in England and Normandy until Richard had completed the third crusade's stated objective of retaking Jerusalem.
Staying in Acre for a few months longer than he actually did, Richard joyfully receives the news of Saladin's death. Taking advantage of the internecine fighting between Saladin's sons and their uncle for control of the Ayyubid dynasty, Richard reestablishes the Kingdom of Jerusalem to something like it's borders after the first crusade. If things shake out right, maybe he eventually even takes Egypt.
Perhaps this just pushes the ultimate collapse of the crusader states back a few decades, but alternatively maybe it leads to substantial amounts of Islamic territory reverting to Christianity. The fourth crusade doesn't happen--or if it does, it gets to Egypt--so Byzantium doesn't fall to the Latins. Instead, it becomes more than a moribund shell of its former self for the next three centuries before ultimately falling to the Ottomans. Obscenely optimistically, this gets us to a 21st century in which Antioch Christians in Syria and Coptics in Egypt constitute majorities of their respective countries' populations.
Less sexily, I could shoot for pretty much the same thing by pulling Frederick Barbarossa out of the Saleph river before he drowned.
- 1600, modern-day Gifo Prefecture. Prior to the decisive battle of Sekigahara, I'd appear--attempting to replicate something similar to Constantine's putative Milvian Bridge conversion experience--to both Ieyasu and Mitsunari, explaining to both in turn that promising to open Japan up to European influence would guarantee each one of them the ability to consolidate the country under his family's rule. If that results in the Tokugawa shogunate settling in as it did or if it leads to an Ishida shogunate filling the void isn't important, so long as the winning side welcomes the Dutch and Portuguese with open arms instead of keeping them at arm's length for decades and decades. Commodore Perry can force his way into somewhere in Southeast Asia instead.
Let's give Japan a two centuries' head start on what history gave her. It'll trickle down to Korea and China. We might be opening a Pandora's box, but maybe a more serious East keeps Europe from ripping itself apart in WWI.
These necessarily presume a Great Man approach to history. While I personally lean a little more in the direction of Herbert Spencer, it doesn't matter much in this context since it's presumably beyond the scope of a single time traveler to engineer meaningful changes of entire social environments. If a single man is going to change history, it's going to have to be by changing the outcomes of the great men of history.
* The specific word choice here is deliberate, since it strikes me as blatantly obvious that travelling backwards in time is impossible. If it were possible, we'd see evidence of it all the time. Yes, in the future it is conceivable that for understandable reasons there would be lots of restrictions on journeying into the past like not allowing anyone or anything to realize you were there, but that's an impossibly high standard to maintain--unless, I suppose, humanity (or whatever would travel backwards in time) had been so completely altered from humans today so as to be characterized by a nature unrecognizable to us in the early 21st Century. They'd all have to be members of the hivemind or surely somewhere some teenager would go out on a time travel joyride without his parents' permission.