Were he alive today, Lee Harvey Oswald could not assassinate a U.S. president in the same manner that he killed John F. Kennedy in Dallas.
Oswald's most obvious issue would be his lack of opportunity.
That's because the Secret Service would strongly recommend against the president moving through Dealey Plaza other than in an armored vehicle. These Secret Service vehicles aren't just bulletproof, they are designed to prevent heavy-caliber round penetration and to ensure the president's safety in the event of a chemical, biological and presumably nuclear attack. Note also that Secret Service vehicles have tinted windows, so as to prevent attackers from spotting the president's car in a motorcade.
What if the president insisted on walking or riding in an open top car?
In that case, the Secret Service would station agents all around him or her so as to obstruct possible rush attacks from the crowd and to make a sniper's job much harder. Moreover, assuming it was a pre-announced rather than unplanned visit, everyone in the crowds the president passed by would have been pre-screened for weapons.
The best example of how the Secret Service deals with these situations is evidenced by an inauguration. As soon as the president steps outside of the vehicle, the agents screen around him from all sides. If the president keeps walking beyond an area the Service is comfortable with, they will politely ask him or her to get back in the vehicle.
Even in situations where security procedures must be less obvious, such as then-President Barack Obama's 2015 Selma memorial march, the Secret Service will employ various means to prevent possible attackers from approaching.
That's not all.
After all, if a president were to walk through Dealey Plaza today, there would still be rifles in the Texas school book depository. But this time they would belong to the elite snipers and spotters of the Secret Service's counter-sniper teams. It wouldn't just be the book depository, most of the buildings overlooking the route would have counter-sniper teams posted on them. In turn, if someone decided to open a window while the president was either approaching or walking past, the response would be immediate.
Here's what roughly would happen.
First, the spotting counter-sniper team would radio the Secret Service command post about the possible threat. The command post would then immediately relay that news to the president's Secret Service detail, who would hold or put the president into a covered position. Simultaneously, a Secret Service ground team would rush to the relevant threat room to arrest the individual.
Yet for the person in the window, that's the best case scenario.
Because were the counter-sniper team to perceive an imminent threat, all the aforementioned would happen, but the counter-snipers would shoot to kill. In his book, Standing Next to History, former Secret Service agent Joseph Petro recounts nearly shooting someone holding a toy gun near a presidential motorcade. His point is well-taken: when it comes to possible threats to the president, the Secret Service does not hesitate.
If these procedures existed in 1963, Lee Harvey Oswald would be far less well-known today.