A stalemate is a situation in which further action is blocked. The stalemate during World War I was broken in 1917 by  the entry of the U.S. into the war.
Explanation:
The war began in 1914 when a young anarchist murdered Archduke Franz Ferdinand of Austria. The German forces occupied Belgium and parts of France, anticipating a quick victory, but the struggle continued and spread to other regions of the world.
Despite their sympathies for Britain, France and their allies, the United States remained neutral in the early years of the war. The commercial ties with the allies remained firm, because the British Navy controlled the seas, blocking the access of Germany to vital goods. Germany tried to break the Allied naval blockade using submarines, with which they sank military, merchants and civilian ships, among them the Lusitania, a cruiser of the Cunard line, in 1915. Among the 1,198 passengers killed there were 128 Americans.
The straw that broke the camel's back was the resumption of underwater war without restrictions by Germany and the interception of the "Zimmerman telegram". The telegram revealed a German plot to help Mexico recover Texas, New Mexico and Arizona if it attacked the United States.
When the United States entered the war in April 1917, the US military had scarcely 130,000 troops, no tanks, and few aircraft. Congress quickly approved mandatory recruitment to swell its forces. A German admiral claimed that few US combatants would arrive in Europe due to submarines blocking their way.
However, they did it. "Lafayette, here we are," said a colonel at the grave in Paris of the French nobleman who helped in the war for the independence of the United States.
The allies were beaten and exhausted after three years of trench warfare. The Americans played an important role in the last year of the war, especially when the German forces launched their final offensive. The arrival of the "en masse soldiers", the members of the United States Expeditionary Force, helped the Allies maintain their lines and break German morale in the final months of the war.
Four million Americans served in the armed forces, two million were shipped to Europe and 1.4 million participated in fighting, helping to get the Germans out in Marne and fighting well-known battles in Cantigny, Chateau-Thierry, Belleau Wood and St. Mihiel.
The war confirmed the leadership role of the United States in international affairs. In the country, it expanded the size and reach of the government and even helped the woman secure the vote after thousands of them entered the armed forces and worked in the factories.
