First World War 1914-18

In 1914, Germany attempted to defeat France in a lightning attack on Paris. The French and British armies mounted a desperate defence, which resulted in a "stalemate". Both sides dug into lines of trenches running from the Belgian coast to the Swiss border - right across Nord-PasdeCalais. Over half of the region was occupied by Germany, close behind German lines. The rest of France was determined to fight on so long as part of their country was occupied - millions of Allied troops died in the bitter trench warfare.
The Germans and their Austrian and Turkish allies had more success against Russia on the eastern front. Russia's failure in the war led to the 1917 Bolshevik Revolution.
In 1918 peace with Russia enabled Germany to bring back forces from the east. They mounted a last-hope offensive attack in N.France, before reinforcements from USA crossed the Atlantic. It almost succeeded.
Nord-PasdeCalais was devastated by years of fighting - its major towns lay in ruins.

More information:
First World War in more detail
Vimy canadian memorial
Arras: French memorials & museums