Answer: Most of the war was fought on its soil.
Step-by-step explanation:
The war began in France with several destructive battles and the German invasion split the country in three: war-front France, occupied France and behind-the-lines France.
The situation for Great Britain and Germany, the other two biggest Western countries at war, was different since only France was both one of the most significant battlefields, and partially occupied by the enemy as well.
Furthermore, France was the main stage of operations of the Western front, and where the result of the war was settled in 1918.