Today's medal is the United States Pennsylvania National Guard Mexican Border Service medal of 1916.
While the world's attention was focussed on the conflict erupting from Europe the United States had a deepening issue on their southern Mexican border. Following the Mexican revolution of 1910 and deposing of Dictator President Porfirio Diaz the pro-democratic businessman Francisco Madero rose to power as Mexican President assisted by key revolutionary figures such as Pancho Villa. The Madero government only lasted 2 years before being ousted in a coup by General Huerta and replaced with a military junta. This regime lasted just over a year before Villa and allies such as Venustiano Carranza forced him out leaving a power vacuum. Villa's influence was such during this period that the United States considered recognizing him as de facto ruler of Mexico.
After his former ally Carranza challenged Villa in 1914 a civil war broke out and with Carranza enjoying US support, defeated Villa's forces. Enraged by this Villa commenced cross-border attacks in New Mexico attempting to goad the US to invade and move against Carranza regime. Villa went on to carry out attacks against border towns in Texas and New Mexico. In response Brigadier General Pershing mobilized the US Army and moved south with the intention of defeating Villa.
In response National Guards units were mobilized across a number of border states including Pennsylvania resulting in more than 100k NG servicemen being stationed on the border in readiness by 1916.
In 1917 the famous Zimmerman telegram was intercepted by the British which requested Mexico to join forces with Germany and directed them to attack the US southern border with the promise of the returning of lands lost to the US. This intercept and the US suspicion of the presence of German smuggling operatives along the border heightened tensions.
In 1918 following a skirmish between US Customs Officals and Mexican border guards US army and Mexican forces converged on the the town of Ambos Nogales resulted in a battle that left the Mexicans defeated. Further skirmishes continued through 1919.
A number of medals were commissioned in the aftermath of the Mexico operation. The regular Army, Navy and Marines received a national Mexican Service Medal while National Guardsmen based within each of the participating states received separate federal Mexico Border Service awards. The example pictured is the specific award by the state of Pennsylvania and has the awardee's service number on the rim. These medals were struck by Philadelphia jeweler Joseph K. Davison & Sons.
This mobilizing of US forces and the valuable battle experience gained by them against the Mexicans set the United States combat units in good stead for their eventual introduction to the European theatre when the United States joined the fight against the Central powers in 1917 with troops arriving on the Western Front in 1918.