A popular game amongst history students is if you could back in time to prevent something happening, what would you do.
By far the most common answer is to stop Hitler, either by killing him before he became the top Nazi or for the more imaginative, preventing his parents ever getting together so he would be born in the first place.
Of course by preventing the second World War and 25 million deaths you would still have the first World War and the 18 million deaths that caused.
The better answer would therefore be to go back and stop whoever or whatever sparked the first world war so that would be Gavrilo Princip who assassinated Archduke Franz Ferdinand when his car stopped outside the cafe he was sat outside of which sparkled off the whole thing which led to the second war.
That's 43 million deaths over the two wars prevented simply by making sure the young Princip went to another cafe that day or delayed enough that he wasn't sat outside when Ferdinand's car stalled.
As with time travel though, things are not that simple because of time ripples and anything that happens in the past changes the course of history.
So let's consider that we go back, find Gavrilo on his way to that cafe and we 'accidently' tip a bucket of cold water over him forcing him to go home and change so he is not sat outside the cafe drinking coffee which means the Archduke remains alive and Austria doesn't declare war on Serbia to start the chain of events which ends with a nuclear weapon being dropped on Japan and a Cold War between East and West.
Everybody happy and instead of trench warfare and a wall built across Berlin, it's sunshine and lollipops.
It would mean of course that the woman's movement would not have happened or at least put back for a few more decades as would the Civil Rights Movement in America while the Soviets would probably not have launched Yuri Gagarin and Vostok 1 until much later which would have delayed Appollo 11 and Neil Armstrong making the giant leap for mankind.
A delayed Space adventure would also mean a delay in satellites so SatNavs, accurate weather forecasts, technical communications and TV beamed around the globe decades would arrive far later than when we did get them.
The internet came around as a way to keep communications open during a nuclear attack but no cold war would mean no need for nuclear safety so the internet may still have came around but not for some time later as would the computer which was invented to aid the breaking German U-boat codes.
Fleming's discovery of penicillin in 1928 was initially over-looked and was only made into an effective drug in WW2 when medical researchers were seeking a method of infection control in troops. It may well have been noted for it's medical properties later but without the war it may well have stayed on the shelf and not in hospitals and medicine cabinets saving lives today.
It is amazing to think how different 2016 would be if one young Serbian rebel rouser had not stopped for a coffee or if a time traveler armed with a bucket of water dampens him before he gets there.