Complex sentence: Prior to the First World War the river formed part of the border between Kingdom of Italy and the Austro-Hungarian Empire and during that conflict was the scene of bitter fighting between the two countries, culminating in the Battle of Caporetto in 1917.
Split into simpler sentences: Prior to the First World War, the river formed part of the border between Kingdom of Italy and the Austro-Hungarian Empire. During WWI, it was the scene of bitter fighting between the two countries, culminating in the Battle of Caporetto in 1917.