Complex sentence: The trench mortar was first used in 1915 during the Battle of Loos to fire a smoke shell, at first it was not liked but as the construction was improved it was widely used and produced in two sizes.
Split into simpler sentences: The trench mortar was first used in 1915 during the Battle of Loos to fire a smoke shell. At first it was not liked but as the construction was improved it was widely used and eventually produced in two sizes.