Why did president wilson withdraw american troops from mexico in 1917?

The expedition began to approach its end when General Álvaro Obregón made a threat that if they did not stop, they would send troops from the Mexican army to cut supplies to US troops in Mexico. And after a confrontation between Mexican and American troops near the border, tension increased. Pershing, furious, requested to attack Chihuahua City, but due to the participation of the United States in the First World War, President Wilson did not accept.

In November 1916, the United States agreed to remove its troops from Mexico. But the American soldiers left Mexican territory until February 1917, and Pershing failed to capture Pancho Villa.

answer; ///the correct answer is,by introducing a merit system of nobility.

sorry i really dont know