The Spanish Empire, often referred to as the Spanish Colonial Empire, was the first of the European empires that began to appear in the course of the Modern Age, it being one of the most important players of Europe for the remainder of the XVI, XVII, XVIII and partially the XIX century.
A common misconception is that of the very adjective of Colonial, something that was spoken by the English, Dutch and French about the Empire in its overseas possessions and became internalized for the next centuries. The proper term of colony (which had changed its meaning from a neutral term to the derogative and negative conception of now) was never given to the territory in America, instead they were referred to as kingdoms, hence the need of Viceroys to administrate the territory.
Often an enemy of France until the rise of the Bourbons, but its longest and most regarded archnemesis was The British Empire, which in its beginnings was also meant to actively dry the Spanish of the gold they got from America.
Isabel and Fernando, The Catholic Monarchs (1470s-1500s)Unified the Peninsula (minus Portugal), undermined the power of aristocrats, renewed the army and put an end to both the Reconquista and the Spanish Middle Ages. Also, they funded a certain Christopher Columbus for an extravagant new route to the Indias.
Why were the Spanish and Portuguese the ones to first sail to the New World? Why not France, England, The Ottoman Empire, or even beyond that, Ming China or the Mughals?
The reasons behind that are what is called amongst historians the Iberian Privilege: in the XIV century, a short cold snap of the European climate affected all of northern Europe, producing shortages of food and famine across those regions and overpopulation of the cities; then came the Black Death, which went from northern Italy across all of the trade network of Europe.
You can guess what happened to the people of the North when plague came with famine.
But the Iberian Peninsula, being in the Mediterranean and at the end of the land roads of trade, didn't suffer that much from the population crisis, instead, as the Reconquista continued, the Iberian kingdoms found themselves richer and with more land. With an undiminished population, their growth was secured. What's more, their position meant they were in permanent touch with the Mediterranean and Northern nautical techniques.
Then came the XV century, this century was marked with the infamous HundredYearsWar between the French and English that started the last century, the inner conflicts that had wracked the Holy Roman Empire, the rise of the Ottomans at the east and the beginning of the isolationist policy of China.
By the latter half of the fifteenth century, the then Queen Isabel chose to marry King Fernando, which was reported to be a love match between the two, and with it, the crowns of the kingdom of Castile, largest in Hispania, and the kingdom of Aragon, joint with the kingdom of Naples; Spain thus became a prime power that not only was strong militarily, but economically.
This union, while it suffered a rebellion, was able to finish the Reconquista once and for all, annexing the territory of Granada, today southern Spain. A major factor is that, due to their crushing of the opposition and their coordination, the Catholic Monarchs were able to wield a great amount power in practice, not just in title.
The monarchs then decided to choose good marriages for their children, deciding to marry their daughter, Juana, to Felipe "the Handsome", a noble of the House of Habsburg whose heritage included the territories once held by the duchy of Burgundy (from which the Spanish used their flags) and the Habsburg possessions.
After the fall of Constantinople, the Catholic Kings financed the expedition of Christopher Columbus, who was rejected by the Portuguese, to find land that would become a part of the crown and that which he would become a direct Viceroy.
Juana I, "The Mad Queen" (1504-1516)Her short reign saw the roots of an ill-fated alliance with England and the HRE, but is best remembered for her going apeshit crazy and getting imprisoned in a tower afterwards by her late husband, who is...
Felipe I, "The Handsome" (1506)Brief as he was, and never getting to reign alone, is just remembered for being a jerk and totally disregarding the queen in his endless affairs, thus starting a long tradition.
Once both Juana and Felipe were offed for different reasons, Fernando took again the crown he shared with the now deceased Isabel and, before dying, left the kingdom of Aragon and Naples as inheritance to the kingdom of Castile, thereby joining the two countries into a single power under a single person.
That person was Charles I of Spain and V of Austria. Last Emperor of the Holy Roman Empire to be crowned by the Pope himself. At the moment of his ascension, the Spanish Conquistadores began to enter into the territories held by the Aztec Empire and the Inca Empire, who took advantage of the fragmentary state of the two states and managed, by turning the very native population against their overlords, conquer most of modern México and Perú.
Carlos I, "The Kaiser" (1516-1556)Arrived to Spain without a grasp of culture, society or language, for that matter and faced quite a few rebellions for it, which he crushed promptly. Protestants were tougher, though, when Martin Luther nailed his theses under his nose. Cue the endless religious wars funded with the extensive resources of the Spanish empire.
Also, when he understood he couldn't win he resigned to his son and retired into a monastery.
Felipe II, "the Cautious" (1556-1598)As the religious wars escalated into political wars, he attempted an alliance with England that twice ended very badly. Also remembered for annexing Portugal. And for allegedly building the Escorial Palace to seal a hellgate, filling it with Catholic art and relics. Incidentally, all kings of Spain after him are buried there.
An archipelago in the Western Pacific was named in his honor.
Felipe III, "the Pious" (1598-1621)Best remembered for expelling all non-Catholics from Spain, namely Muslims. And gypsies.
Felipe IV, "the Great" (1621-1665)Under the advice of the Count Duke of Olivares, tried to extend the war effort from Castile to the whole realm, which prompted rebellions in Portugal and Catalonia that led to the former's independence and the latter's annexation to France. Catalonia was re-taken, albeit cut to a size, Portugal didn't.
Carlos II, "the Bewitched" (1665-1699)Born with serious deformities both physically and psychologically (He was deemed as "retarded"), he was not very able to reign, and was also sterile. And, apparently, he was aware of how much of a failure he was regarded as. His death meant war.
The House of Bourbon
Felipe V, "the Animous" (1700-1746)The Crown cost him Gibraltar and Minorca, of which only the latter was re-taken in a later war. Also, he famously suppressed medieval rights as a punishment for supporting the other guy in Aragon (namely, Catalonia and Valencia), which started a long tradition of animosity between the Bourbons and those regions.
Luis I, "The Beloved" (1724)You would be hard-pressed to find any reference to him in general history books, as he didn't last a year as a king. After his death, his father took the crown again.
Fernando VI, "The Fair" (1746-1759)Not very often remembered, if at all. Didn't wage wars, nor alliances, nor anything. His reign was mostly domestic and peaceful, which, apart for being unusual for a European country of the time, doesn't grant many pages in history books.
Carlos III, "The Mayor of Madrid" (1759-1788)Best remembered for his somewhat unusual interest in urbanism, he reformed most of Madrid into the neo-Classical style for which it is known.
Carlos IV, "The Hunter" (1788-1808)He was more interested in clocks than politics, too bad The French Revolution and, later, a certain Napoléon Bonaparte were knocking into the door as his own scheming son wanted his head on a spear. Although at least he ended better than his French cousin, who also was an amateur clockmaker.
The House of Bonaparte
José I, "Pepe Bottles" (1808-1813)Imposed by his brother, Napoleon, he was disliked by near everyone from the Spanish America (which declared independence so they hadn't to have him as king, mind you) to peninsular society, kickstarting an insurgence war that bleed French resources and, by the time the Russian campaign kick off, led directly to the downfall of the first French Empire.
The First Restoration of the House of Bourbon
Fernando VII, "the Desired", aka "the Felon King" (1813-1833)Crushed the Spanish liberals, abolished the constitution his Spanish supporters approved on his absence, empowered American revolutionaries by denying the colonies the seats on the parliament the aforementioned constitution granted them on the envisioned constitutional monarchy, re-implanted The Spanish Inquisition, abolished by said liberals, inadvertently granted independence to the American colonies by upsetting the military intended to fight the revolutionaries and getting them to revolt against him, swore the previously abolished constitution when they managed to corner him and secretly schemed with foreign authoritarian powers to crush them once and for all. Which he did. After that, followed ten years known as the "ominous ten years". In his deathbed, he changed his last will so his daughter could reign (a very liberal standpoint, for a king that fiercely clung to authoritarian, medieval-like monarchy) instead of his even-more-authoritarian brother. Widely considered one of the worst and most faithless kings in history.
Isabel II, "That of the sad fate" (1833-1868)Crowned as a child, married to a gay man. Known as the Nymphomaniac Queen. Her reign is a succession of civil wars against the supporters of her father's brother, which forced her mother, and then herself, to rely on the liberals her father so fiercely hated to stand on the throne as sleazy politicians turned into the office - and, allegedly, her bed.
It is commonly regarded that she started as an Spoiled Brat who didn't quite have a grasp on how to rule a country and relied - quite effectively - on the military as chancellors and advisers, as well as, well, military leaders against her enemies. As she grew, she was stuck in a loveless marriage and got fat and was taken advantage of by said sleazy politicians who saw her need for affection, sometimes through sex, as an opportunity to gain power and influence, which gathered a general mockery from the population as her popularity decreased through - rightfully - perceived institutional corruption until she was finally kicked out of the throne by 1868.
The House of Saboya
Amadeo I, "The Elected" (1870-1873)
After Isabel II was deposed, debate about who the king of Spain might be started a controversy which even bleed into the international sphere with a war between France and Prussia. The Parliament eventually decided Amadeo, from the family that successfully had united Italy, would be a nice king. This option was strongly supported by prestigious general Juan Prim.
Sadly, Juan Prim was murdered shortly before Amadeo arrived to Spain, and the new king was received with utter contempt. The mocked him endlessly, openly and in front of him (what with celebrating a parade for the new king and filling the front rows with the ugliest, oldest prostitutes they could find) when not blatantly ignoring him, belittling him as unmanly or just plain bullying him.
The Second Restoration of House Bourbon
Alfonso XII, "the Peacemaker" (1874-1885)
Once Spain had rendered into utter chaos, he arrived with a high popularity to boot. His new regime was engineered by Antonio Cánovas del Castillo, who was a well known anglophile and designed a parliamentary system akin to that of the United Kingdom with the exception that Cánovas expected little from the Spanish population and he was doubtful they could be civilized enough to implement a full democracy without civil unrest arising. So he arranged with local hicks known as caciques for the falsification of election results so "elected governments" can be stable and predictable. Hence, a two party system took turns exactly every fourth years, one of them - the Conservative Party- lead by himself and the other one - the Liberal Party- led by Práxedes Mateo Sagasta while keeping undesirable factions (namely, PSOE, the Socialist Party) out of the system.
Alfonso XIII (1885-1931)
By the time the Spanish-American War kick off, Cánovas had been murdered the year before the conflict started and Sagasta took off every responsibility for the humiliating defeat, dying in 1903. Alfonso XII died prematurely and the young king was actually too young to rule over the crisis.
Sadly for him, the two-party system was steadily falling apart once their masterminds had passed on and their successors lacked both charisma and political skills to keep it. The impact the defeat against the United States had on the population further eroded the establishment. And it got worse.
At request of U.S. President Theodore Roosevelt, Spain held an international summit between France, Germany, the United Kingdom and others in Algeciras, Andalusia in 1906 which actually prevented World War I for the next 9 years. After that, Spain got interested in the colonisation of Northern and Southern Morocco which was implemented with extreme prejudice, and resulted in an unprecedented bloodshed. Crime skyrocketed and benefits from commerce due to Spain's neutrality in World War I didn't translate into a better living conditions for workers, which led to a bloodbath in Catalonia. Spain's North African adventures triggered a further crisis in 1921, when Manuel Silvestre led a large but poorly-equipped army into Morocco's Rif Mountains; they were massacred by Abd el-Krim's rebels at the Battle of Anual, initiating the Rif War.
This combination of military disasters and economic hardship destabilized the Spanish government. Spanish governments lasted mere days, and the situation was so desperate that the King relied to the military in 1923, which led to a dictatorship that year under General Miguel Primo de Rivera, the same year a certain Benito Mussolini seized power in Italy.
Primo de Rivera accomplished most of his stated goals by cutting crime, striking seriously Abd-el-Krim rebels in the Moroccan campaign and patching social welfare issues, but in the way he made enemies of the PSOE (Socialist Party), the PCE (Communist Party) and the whole of the elites in both Catalonia and the Basque Country, who turned to nationalism. And then, in 1929, The Great Depression finished him.
The King promptly fired him, but his reputation had bleed into the crown, and in 1931, an election held gave majority to left-leaning and republican parties, and he chose to leave the country in order to prevent a civil war for breaking in. It didn't work. The Spanish Republic was established, but it didn't last.
The Third Restoration of House Bourbon
Juan Carlos I (1975-2014)In 1947, Franco had declared Spain was a kingdom again, but being as he didn't like any of the claimants (particularly Juan, the heir to Alfonso XIII's throne after his elder brothers renounced their rights, and whom he considered too liberal even though he tried to fight on the Nationalist side during the war) he held off his decision until he picked Juan's eldest son in 1969, considering him to be the perfect tool to continue his regime after his death.
However, Juan Carlos was shrewd enough to know which side he should pick, and for a long time he had buttered Franco making him believe he would just allow things to keep up while he planned for the restoration of democracy. Finally, in November 1975, Franco died (and, yes, he is still dead) and he was crowned as Juan Carlos I.
Initially, he was not very popular, as he was still seen as pretty much Franco with a new face, and the leader of the Spanish Communist Party, Santiago Carrillo, famously claimed that he would be nicknamed "The Brief".
Instead, with the support of newly-picked Prime Minister Adolfo Suárez, Juan Carlos managed to ensure the Francoist parliament would vote itself out of existence, all political parties were legalized (even the Communists, in a tricky moment where the military leadership nearly rose again) and eventually elections (the first in more than forty years) were called for Constitutional Courts. The Constitution was accepted in referendum by a majority of the Spanish people, and a new democratic system was finally implemented.
Then, in 1981, a group of Civil Guard men invaded the Congress during the voting on who should replace Adolfo Suárez (who had recently renounced to his position) as Prime Minister, with the intention of being the first step to restore the dictatorship, while a general sent the tanks into the streets and tried to convince others to follow. Then the King appeared on national TV, wearing his uniform as Captain General of the Armies (the highest rank in the Spanish Army) and ordered the Armed Forces to support the democratic system. The coup was defeated soon, and Santiago Carrillo (yes, the same guy as before) was heard saying "God Save the King!"
During the next years, he would be one of Spain's foremost representatives in foreign countries, while several Prime Ministers (after Adolfo Suárez came Leopoldo Calvo Sotelo (1981-82, Felipe González (1982-1996), José María Aznar (1996-2004), José Luis Rodríguez Zapatero (2004-2011) and Mariano Rajoy (2011-2018)) held the reigns of the government. In 2007, during the Hispanoamerican (Spain, Portugal and the parts of America that were colonies of both) Conference: while Zapatero was speaking, Hugo Chávez of Venezuela started to interrupt him, calling the previous Spanish Prime Minister, Aznar, a "fascist" and more. Fed-up, the King shouted at him "¿Por qué no te callas?" (Why don't you shut up?)note , a sentence that gained instant Memetic Mutation status.
However, in the later years, his popularity started to take hit after hit: his eldest daughter divorced, his younger daughter and her husband got involved in several scandals related to misappropiation of public funds, and, most infamously, he broke his hip while hunting elephants in Botswana, which particularly angered the Spanish people since they were in the middle of a very tough recession that had left hundreds of thousands of people without a job. In the end, due in part to both these scandals and his advancing age, he decided to renounce and pass the crown to his only son. And then, after a scandal involving him receiving kick-backs for aiding in the construction of a high-speed railway in Saudi Arabia, he decided to go into self-imposed exile in August 2020.