As at the end of 1915, the Germans were winning in WW1 because the German army opened the Western Front which aided the invading of Luxembourg and Belgium. They also gained military control of strategic industrial areas in France.
The Western Front was one of the strategic theaters of war during the World War 1. On the onset of the war in August 1914, the German military opened the Western Front by invading Luxembourg and Belgium, they continued by overtaking strategic industrial regions in France.
This military offensive was halted at the Battle of the Marne.
Both sides in the war resulted in the digging of trenches. To overcome this tactics of trench warfare on the Western Front, both sides tried alternative new military tactics which included poison gas, aircraft, and tanks.
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