Serna was born in the City of Chihuahua, in the Mexican state of Chihuahua, to a poor family. In 1916, when he was 20 years old, Serna decided to enter the United States, by crossing the Rio Grande and going to El Paso, Texas in search of a job and better way of life. Serna did not know how to speak English and he was only able to find low-paying jobs. He soon found himself working as a farm hand in a sugar beet field in Denver, Colorado.
The United States declared war on Germany on April 6, 1917. Serna was with a group of men in Denver, when he was picked up by federal officials. He faced the possibility of being deported, but before that could happen, he volunteered to serve in the Army.
World War I
After his basic training Serna was sent overseas and assigned to Company B, 355th Infantry of the 89th Division. When the Army realized that he was a Mexican national, he was offered a discharge. Serna, however refused and chose to stay with his new friends. Serna's unit was ordered to begin an advance towards the Meuse River and Argonne Forest in France. When his unit arrived at Saint-Mihiel, they encountered a German machine gunner who killed 12 soldiers. Serna crawled up to the machine gunner's left flank. Even though his helmet was hit twice with bullets, Serna was able to get close enough to throw four grenades into the nest. Eight Germans surrendered and the rest in the nest were dead. On September 12, 1918, Serna's unit was engaged in combat inside the Meuse-Argonne region when he spotted a German sniper and wounded him with his Enfield rifle. Serna followed the wounded soldier to a trench. He threw three grenades into the trench which resulted in the death of 26 enemy soldiers and the capture of 24 prisoners. Serna was wounded in both of his legs by sniper fire, four days before the Armistice. During his recovery, General John J. Pershing, Commander-in-Chief of the American Expeditionary Forces, pinned on his chest the Distinguished Service Cross, the second highest military decoration of the United States Army to the Medal of Honor. Serna was told by an officer that "Buck Privates" were not eligible for the Medal of Honor, and that he did not know enough English to be promoted. The officer in question was wrong because Private David B. Barkley who also served in the 89th Division, was awarded the Medal of Honor for his actions. It so happens that years later it was discovered that Barkley was Hispanic, thus the only Hispanic recipient of the Medal of Honor in World War I. Field Marshal Ferdinand Foch, Supreme Commander of the Allied troops, awarded Serna the French Croix de Guerre for bravery.
