The past has teeth.
If you're not sure what this phrase means, read 11/22/63 by Stephen King, and you will see how the recalcitrant past bites those who try to change it. King's novel, a masterpiece of time travel nuanced by nostalgia, is a full-size exploration of character, action, reaction, and repercussion.
The main theme of 11/22/63 is that certain patterns are entrenched in our lives and will not change. Jake teaches English in both present and past; likewise, people he meets are similarly named and fated, whether in 1960 or 2011. For example, on an early trip through the time portal, he saves a young girl from a paralyzing accident in 1958 and checks for repercussions in the present day. From what he can tell, her alternate life turns out much like her real life, except she seems to have achieved more when paralyzed.
However, try to circumvent more fates, and the past becomes more wolf-like, hungry for blood. Dead batteries, wrecks, muggings~the past stops at nothing to keep from being altered. King's concept of reality (explained to Jake by the eerie "yellow card men" outside the time portal ) is that of a puppet jerked around on strings of occurrences. When a past event is changed, it multiplies into alternate strings, which then become tangled. And even just by stepping foot into the past, Jake creates an alternate string of events which affect everything from that time forward. Changing such a major event as John F. Kennedy's assassination might create a snarled knot of threads, riotous enough to strangle the puppet of reality altogether.
Patterns and the intransigence of time: two lovely themes. How often do we ask ourselves those weighty words~what if? What if I hadn't gotten in the car that day, gone to a different school, spoken up instead of sitting mute? My life might have been totally different. But would it have been that different? Perhaps, the quotidian simply resets to some personal default: similar people, places, things no matter where you go or what you do.
Instead of finding patterns dull, I now see them as comforting signs, that I am where I am supposed to be. Attend to these patterns, these recurring themes, and you might just start to see what animates your puppet.
Does Jake Epping save the President on November 22, 1963? Ohhhh, you'll have to read the book to find out. Let me just say, having led the reader to that murderous point for the entire novel, Stephen King will not disappoint you.