Military Service in Mexico (in Spanish, Servicio Militar Nacional, or SMN) currently involves all males reaching the age of eighteen years. Selection is made by a lottery system using the following color scheme: those who draw a black ball must serve as "availability reservists", that is, they must not pursue any activities whatsoever and receive their discharge card at the end of the year.
The ones who get a white ball serve in a Batallón del Servicio Militar Nacional (National Military Service Battalion) composed entirely of one-year SMN conscripts. In certain cities, such as Veracruz and Ciudad Madero, the lottery system also includes a blue ball; whoever draws it must serve in the navy. Military service is also (voluntarily) open to women.
Conscripts are trained in basic combat with fire weapons, in one Infantry Battalion.
Their formation was brought about after German submarine attacks against Mexican oil tankers that were providing fuel and materiel to the Allies. These attacks eventually caused the Mexican government to declare war on Germany. Mexico sent an expeditionary force, the Escuadrón 201, to fight in the Pacific War.