There are three schools of thought concerning this: One is that modern history is from 1950 onward. Another is that modern history is from 1899 onward The final one is that modern history doesn't...
1789 was the start of modern history. :)
Personnaly I would go back a lot further, to the Renaisance. All history is a continuum, but certainly there are some eras where things did take off rapidly....