Why do the Germans have a limited window to win the war in Spring 1918. (WW1)

The answer is quite simple: USA. Americans had refused to get involved in what they saw as a purely European war, fought between European colonial Empires for European territorial gains. President Woodrow Wilson had always strove to uphold a policy of strict neutrality during the conflict. However, in 1915, a German U-Boat sank the transatlantic British commercial cruiser Lusitania which resulted in the death of several passengers including 128 American citizens. Wilson did not retaliate but demanded that all attacks to passenger, civilian ships cease and the German Empire accepted in order to avoid the entry of the USA in the war in favor of the Allied Powers. In 1917 the Germans decided to reverse this policy and started attacking civilian passenger ships. They did it because after the Russian Revolution, Russia was no longer fighting in the war against the Central Powers. This allowed the Germans to transfer all their men and materiel from the Eastern front to the Western front, strengthening their military position. Since they knew that such move would inevitably cause the involvement of Americans in the war against the Central Powers, Germany sent a telegram to their ambassador in Mexico (the Zimmerman Telegram) offering Mexicans the chance to recover part of the land they had lost to the Americans during the previous century if they accepted to declare war on the US with German logistic support. The message was intercepted by the British and was made public, causing quite an outrage in the US. Germany then sank several US merchant vessels and that finally made Congress declare war on Germany. The Germans knew that it would take more or less 12 months for a large American army to be drafted, trained, equipped and sent to Europe, so they were trying to obtain a decisive victory before the actual entry of American forces in the war.