The Schlieffen Plan was a strategic plan called Aufmarsch I West made by Count Alfred von Schlieffen and the German General Staff. It was made for the army of the German Empire in 1905. It was designed for a war between France on one side and the German Empire, Austria-Hungary, and Italy on the other. Germany and her allies would invade France through Belgium. With help from her allies, Germany would have just enough men to beat France in a few months.
When Schlieffen retired in 1906, Helmuth von Moltke the Younger took over the German Empire's war planning. Moltke got rid of the plan because he was sure that the Russian Empire and the United Kingdom would help France and that Italy would not help Germany. Moltke was right about this. When World War I started Germany and Austria-Hungary had to fight Russia on the Eastern Front and Italy did not help Germany.
Even though he did not have enough men to beat France, Moltke still attacked France through Belgium. He did this because he thought attacking was always better than defending. Moltke was sure that the Russo-Japanese War proved this because Japan always attacked, and Japan won.
Schlieffen believed that defending was usually better than attacking. Schlieffen said that a defender's men can ride trains to a place faster than an attacker's men can walk there. This meant that the defender would always have enough men to stop the attacker. Schlieffen also said that trenches, machine guns, and barbed wire would help the defender a lot. Schlieffen was right. In WWI, defense was always better until attackers used lots of artillery to help their infantry.
A similar idea to the Schlieffen Plan was used by Hitler's generals Erich von Manstein and Heinz Guderian in World War II. In that war, Germany invaded France by attacking Belgium and The Netherlands. Because in the Schlieffen Plan Germany attacked mostly through northern Belgium, France thought Germany would do this again. France put most of her soldiers in north Belgium. But Germany invaded mostly through South Belgium. German troops marched to the sea and trapped half of the French Army in northern Belgium. Because the trapped French troops were starving and could not get any more food, they surrendered. France tried to continue fighting but was too weak to resist and surrendered.