The Spanish Armed Forces are in charge of guaranteeing the sovereignty and independence of the Kingdom of Spain, defending its territorial integrity and the constitutional order, according to the functions entrusted to them by the Constitution of 1978. They are composed of: the Army, the Air and Space Force, the Navy, the Royal Guard, and the Military Emergencies Unit, as well as the so-called Common Corps.

Spain occupies a prominent position in the structure of NATO, which it joined in 1982. Nevertheless, it spends less than the minimum of 2% of GDP on defence recommended by NATO.

Spain has the oldest Marine Infantry in the world and the oldest permanent military units in the world: the Infantry Regiment "Inmemorial del Rey" No. 1 and the Infantry Regiment "Soria" No. 9.

After the long Reconquista ending in 1492, Spain evolved into Europe's foremost power with the voyages of Christopher Columbus the same year, leading to Spain acquiring vast lands in the Americas and conquering a number of overseas civilizations in the decades to come. The period of reconquest of Iberia had ended, and now Spain entered an era of overseas conquest spearheaded by conquistadors. The conquest of the Aztec Empire, the conquest of the Inca Empire and the conquest of the Philippines ensued propelling Spain to the foremost military superpower of the time. Spain was also victorious over the French in the Italian Wars, annexing southern Italy. During the reign of Charles V and Philip II, Spain reached the peak of its power with the Spanish Empire spanning 19.4 million square km of the Earth's surface, a total of 13% being the first empire which the Sun never sets. By the mid 17th century Spain had been militarily weakened by the Thirty Years War, the Eighty Years War along with financial problems, and the lack of reforms, though still holding firm to the bulk of the American Continent.

During the 18th century the new Bourbon dynasty revived Spain's economic and military power through a series of important reforms in the armed forces and the economy, notably those of Charles III of Spain. Thanks to these reforms, Spain performed well during the war of Jenkins' Ear defending overseas territory, won the war of Austrian Succession but had mixed result during the Seven Years' War. Spain had also led successful campaigns in the American Revolutionary War. Spain had recovered considerably by the outbreak of the French Revolution, by 1790, the Spanish Empire was the largest empire in the world. The occupation of a great part of Spain by the French during the Napoleonic Wars resulted in Peninsular War, which was characterized by use on a large scale of guerrilla troops, made necessary by the war's devastating effect on the Spanish economy. Although victorious in the Peninsular War over Napoleonic France, the Spanish military was in poor condition and political instability resulted in the loss of most of Spain's former colonies, who had rebelled against Spanish rule in the Spanish American wars of independence, except Cuba, Puerto Rico, and the Philippines. These too would be lost later in the Spanish–American War.

In the 20th century, the Spanish armed forces did not intervene in the First World War (neutrality) or in the Second World War (non-belligerent), although they did intervene in the Spanish Civil War and in some colonial conflicts. After the arrival of democracy in 1978, they underwent a strong modernization process, becoming modern armed forces. In 1982 Spain entered NATO.

Recently, in the last decades of the 20th century and the first decades of the 21st, Spanish troops have participated together with their Western allies in operations such as Gulf War, NATO bombing of Yugoslavia, KFOR, war on terror, 2011 military intervention in Libya, Combined Task Force 150 and UNIFIL.

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