There were a series of unification wars in the mid 1800s that culminated in 1870. Over the series of wars the Kingdom of Piedmont Sardinia conquered the rest of modern Italy, and united it under their monarchy.

If you'd like to learn more about the period with a casual approach, I recommend Talking History: The Italian Unification, a podcast by Benjamin and Adam Ashwell. It's great. For something more in depth, try The Pursuit of Italy, by David Gilmour.