Pietro Toselli joined the army at a young age and was commissioned second lieutenant of artillery in 1878.

After finishing the tree year curriculum for artillerymen he opted for additional training in military engineering.

[5] When the Royal Italian Army invaded Tigray, Toselli was sent with 1,450 soldiers, 700 militiamen, and four cannons to occupy and fortify the forward position at Amba Alagi.

On the 7 December 1895 the Battle of Amba Alagi began when the position came under attack by the troops of Makonnen Wolde Mikael.

Toselli was by then exhausted and could not move on with his men, he died from a gunshot wound near Bet-Mariam when the advancing Ethiopians reached his position.