[5] After Mexican independence in 1821, the military played an important political role, with army generals serving as heads of state.

[6] Following the collapse of the Federal Army during the 1910–1920 Mexican Revolution, former revolutionary generals systematically downsized the size and power of the military.

The Spanish crown established a standing military in the late eighteenth century to shore up the defense of New Spain against foreign attacks.

Royal army officer Agustín de Iturbide changed sides and made a pact with insurgent general Vicente Guerrero, bringing about independence.

The conservative military and the Roman Catholic Church allied in an unsuccessful attempt to oust the liberal reformers in a civil war.

An important liberal military leader against the French and their conservative Mexican collaborators was General Porfirio Díaz.

Revolutionary generals were unable to come to a power arrangement after their victory over Huerta, plunging the country into a new stage of civil war.

During this same period, these generals, especially Obregón, Plutarco Elías Calles, and Lázaro Cárdenas, systematically downsized the overall size of the armed forces and drastically reduced its share of the national budget, while at the same time creating a professional and largely apolitical junior officer corps.

There is no set number of zones within a region, and these can therefore be tailored to meet operational needs, with a corresponding increase or decrease in troop strength.

The Navy maintains significant infrastructure, including naval dockyards that have the capability of building ships, such as the Holzinger class offshore patrol vessel.

The Naval Infantry are responsible for port security, protection of the ten-kilometer coastal fringe, and patrolling major waterways.

Today, Rural Defense teams work with both local law enforcement and the National Guard towards the goal of hindering organized crime and the threat of the drug cartels.

Each minister serves in a dual capacity: as a full cabinet member reporting to the President, and as the operational commander of their branch, but because of politics and rank, the Navy is subordinate to the Army.

This subordination has allowed the Army to identify its organization as the "Secretariat of National Defense" (Secretaría de la Defensa Nacional—SEDENA).

However, both Presidents Ernesto Zedillo (1994–2000) and Vicente Fox (2000–06) strayed from precedent and reached down to the junior levels to select "more progressive" officers to lead the forces.

Since the year 2000, however, with the economic boost that the country has experienced, the defense budget was decreased to 0.5 percent of the GDP, and in 2007 had an annual expenditure of US$4 billion.

[13] The Mexican Army works around three preparedness missions, or plans: DN1: Preparation of the military forces to repel external aggressions.

The last time this was invoked was in 1942, to send an expeditionary force to the Philippines, after war was declared against Germany and Japan, following the sinking of two Mexican ships by U-boats.

For example, between August and November, military forces are sent to Mexican coastal areas to aid the public in the event of hurricanes or floods.

More recently, the DN3 plan was invoked in 2009 when an epidemic of swine flu threatened the population and in 2010 after the states of Veracruz, Tabasco, Nuevo León, Chiapas, Oaxaca and Guerrero were severely affected by floods caused by a number of hurricanes and tropical storms.

Officer candidates for the three services are trained in military colleges; Mexico City for the Army, Guadalajara, Jalisco, for the Air Force, and Veracruz, for the Navy.

If they choose to do so, this 2nd term would become final, unless they undertake mandatory exams and tests to become corporals, or apply to study in any of the available Military Specialist Technical Schools or for sergeant in the E.M.C.A.

Most conscripts will have received at most only one marksmanship session at a rifle range by the time they have completed their service and are not integrated nor operate with regular army units.

The drafted men attend and participate in weekend sessions, which are of social service in nature, with an emphasis being placed on education, history, physical fitness, and military discipline for one complete year.

Article 129 of the 1917 Political Constitution of the Mexican United States establishes that: "No military authority may, in time of peace, perform any functions other than those that are directly connected with military affairs", but the Army's temporary replacement of civil police forces, in specific cases, before the creation of the Federal Police, has been much debated in Congress and in the mass communications media.

In a similar way in Article 26: "No member of the army shall, in time of peace, be quartered in private dwellings without the consent of the owner, nor may he impose any obligation whatsoever.

In time of war the military may demand lodging, equipment, provisions, and other assistance, in the manner laid down in the respective martial law."