Answer:
By the time the Guerra Bianca, or White War, ended in 1917, Italian and Austro-Hungarian soldiers fighting for three years in extreme conditions at altitudes of up to twelve thousand feet.
Explanation:
The underlined part is a fragment rather than a complete sentence.
This is one way to fix it:
Italian and Austro-Hungarian soldiers had been fighting for three years in extreme conditions at altitudes of up to twelve thousand feet.
