Although you had tagged this question with “London” that is not the correct answer. The correct answer is Paris. The German army came very close to Paris in the opening days of World War I.
The Germans had a clear plan for how they wanted to attack France. They had a relatively weak force in the south and did not attack there with any vigor even though that is where Germany actually bordered France. Instead, they massed a large force in the north and attacked very strongly through Belgium and into France from that direction. Their attack almost succeeded in capturing Paris. However, they were stopped. In what was called the Battle of the Marne, the French were able to stop the German advance. Soon after that, the trench warfare that characterized WWI on the Western front set in.
The answer is "Paris"..Since the German army had already invaded Belgium and North- Eastern France, they had managed to reach within 30 miles of Paris. Their advance was in pursuance of the aims of the Schlieffen Plan, whose primary focus was the swift defeat of France in the west before turning attention the Russian forces in the east. the German army had encircled Paris from the East but was stopped by France..
The battle had ended any hopes Germans had of bringing the war on Easten front to an early close!!!! Hope that answers the underlying theme of the question...