Gaius Caligula (AD 12–41)

There are plenty of other contenders for worst Roman Emperor – Nero and Commodus for example – but  Caligula's mad reign sets a high standard.

After a promising start to his reign he seems to have set out specifically to intimidate and humiliate the senate and high command of the army, and he gave grave offence, not least in Jerusalem, by declaring himself a god; even the Romans normally only recognized deification after death.

Caligula instituted a reign of terror through arbitrary arrest for treason, much as his predecessor Tiberius had done; it was also widely rumored that he was engaged in incest with his sisters and that he lived a life of sexual debauchery, and this may well be true. The story of his making his horse a consul, meanwhile, may have been exaggerated, but it was not out of character.

 Caligula’s unforgivable mistake was to jeopardise Rome's military reputation by declaring a sort of surreal war on the sea, ordering his soldiers to wade in and slash at the waves with their swords and collecting chests full of seashells as the spoils of his ‘victory’ over the god Neptune, king of the sea and by his failed campaign against the Germans, for which he still awarded himself a triumph. He was assassinated by the Praetorian Guard in AD 41.

 Pope John XII (954–964)

Even by the lax standards of the medieval papacy, John XII stands out as a disaster of the highest order. He was elected pope at the ripe old age of 18 as part of a political deal with the Roman nobility, and he inherited a conflict for control of Italy between the papacy and the Italian king Berengarius.

 John had the support of the powerful German emperor Otto I, who swore to defend John's title, but John himself was too taken up with a life of drunken sex parties in the Lateran to care too much either way. He recovered from his hangover enough to accept Otto's oath of undying loyalty and then promptly linked up behind Otto's back with his enemy, Berengarius.

 Understandably annoyed, Otto had John overthrown and accused, among other things, of simony (clerical corruption), murder, perjury and incest, and he replaced him with a new pope, Leo VIII. However, John made a comeback and had Leo's supporters punished ruthlessly: one cardinal had his hand cut off and he had a bishop whipped.

 Full-scale war broke out between John and Otto, until John unexpectedly died – in bed with another man's wife, or so rumor had it.

 King John (1199–1216)

The reign of  King John is a salutary reminder that murder and treachery may possibly be forgiven in a monarch, but not incompetence.

 John was the youngest and favorite son of Henry II, but he had not been entrusted with any lands and was mockingly nicknamed John Lackland. He tried unsuccessfully to seize power while his brother Richard I was away on crusade and was sent into exile upon Richard's return.

 On his accession John had his own nephew Arthur murdered, fearing Arthur might pursue his own, much better, claim to the throne, and he embarked on a disastrous war with King Philippe-Auguste of France in which he lost the whole of Normandy. This singular act of incompetence deprived the barons of an important part of their power base, and he alienated them further with arbitrary demands for money and even by forcing himself on their wives.

 King Richard II (1377–99)

Unlike  Richard III, Richard II has good reason to feel grateful towards Shakespeare, who portrayed this startlingly incompetent monarch as a tragic figure; a victim of circumstances and of others' machinations rather than the vain, self-regarding author of his own downfall he actually was.

 Learning nothing from the disastrous precedent of  Edward II, Richard II alienated the nobility by gathering a bunch of cronies around him and then ended up in confrontation with parliament over his demands for money.

 His reign descended into a game of political maneuver between himself and his much more able and impressive uncle, John of Gaunt, before degenerating into a gory grudge match between Richard and the five Lords Appellant, whom he either had killed or forced into exile.

 Richard might have redeemed himself by prowess in war or administration, but he possessed neither. Henry Bolingbroke's coup of 1399, illegal though it no doubt was, brought to an end Richard's disastrous reign. Richard II has his defenders nowadays, who will doubtless take issue with his inclusion in this list, but there really is very little to say for him as a ruler.

Ivan IV ‘the Terrible’ (1547–84)

Prince Ivan Vassilyevitch grew up at the hazardous court of Moscow, his life often in danger from the rivalry of the boyars – nobles. It gave him a lifelong hatred of the nobility and a deep streak of ruthless cruelty – aged 13 he had one boyar eaten alive by dogs.

 Ivan was Prince of Muscovy from 1533, and in 1547 he was crowned Tsar (Emperor) of all Russia – the first ruler to hold the title. He crushed the boyars, stealing their lands to give to his own followers; he also condemned millions of Russians to a permanent state of serfdom.

Ivan took a vast area of Russia as his personal domain patrolled by a mounted police force with carte blanche to arrest and execute as they liked. Distrusting the city of Novgorod he had it violently sacked and its inhabitants massacred, and he embarked on a disastrous and ultimately unsuccessful series of wars with Russia's neighbors.

 Ivan beat up his own pregnant daughter-in-law and killed his son in a fit of rage. Ivan was in many ways an able ruler, but his ruthlessness, paranoia and taste for blood earn him his place in this list.

 Mary, Queen of Scots (1542–67)

We are so familiar with the drama and tragedy of Mary, Queen of Scots reign that it is easy to overlook the blindingly obvious point that she was absolutely useless as queen of Scotland. Admittedly, ruling 16th-century Scotland was no easy task, and it was made harder still for Mary by the stern Presbyterian leader, John Knox, and her violent, boorish husband, Lord Darnley.

 Nevertheless, Mary showed none of her cousin  Elizabeth's political skill in defusing religious or factional conflict, and she headed into pointless confrontation with Knox and the Presbyterians. At a time when female rule was generally regarded with suspicion in any case, she played up to the stereotype by appearing to live in a cozy world of favorite – including her unfortunate Italian guitar teacher, David Rizzio.

Mary’s suspected involvement in the spectacular murder of Darnley on 10 February 1567 was a political mistake of the first order; her marriage three months later to the main suspect, the Earl of Bothwell, was an act of breathtaking stupidity. It is hardly surprising that the Scots overthrew Mary and locked her up.

Emperor Rudolf II (1576–1612)

Some historians are kinder to Rudolf than in the past, but by any standards he was a disastrous ruler. He was elected Holy Roman Emperor in 1576, though he was prone to long bouts of deep depression and melancholia and he spent most of his time dabbling in alchemy and astrology.

 A staunch Catholic, Rudolf tore up the religious settlement that for the past 20 years had kept Germany's Catholics and Protestants from each others' throats, and embarked on a crusade to eradicate Protestantism from Germany's towns and villages.

 When the Protestants formed a self-defense league, the Hungarians rose in revolt and the Turks launched an offensive, Rudolf shut himself up in Prague Castle and refused to speak to anyone. Eventually the Habsburgs had to agree to replace Rudolf with his brother, Matthias, who restored the religious peace in Germany and signed treaties with the Turks and Hungarians, only for Rudolf to fly into a rage and start up the Turkish war again.

 Rudolf reluctantly signed the letter of majesty granting freedom of worship to Protestants in Bohemia but then embarked on a program of persecution. The Bohemians appealed to Matthias for help, and in 1611 Rudolf was forced to hand power over to his brother. He died a year later, having laid the foundations for the disastrous Thirty years War that would tear Europe apart within six years of his death.

 Having escaped, she was mad to throw away her advantage by going to England, where she could only be regarded as a threat, instead of to France, where she would have been welcomed with open arms.

 Queen Ranavalona I of Madagascar (1828–61)

At a time when the Europeans were spreading their colonial holdings around the world, Queen Ranavalona was able to keep Madagascar free of British and French control, but she did so by establishing a rule so ruthless that it has been estimated that the population of her kingdom was halved during her reign.

 Queen Ranavalona maintained her power by retaining the loyalty of the Malagasy army and imposing regular periods of forced labor on the rest of the population in lieu of taxation. On one notorious occasion she organized a buffalo hunt for herself, her nobles and their families and followers, and she insisted that an entire road be built in front of the party for them all to advance to the hunt in comfort: an estimated 10,000 people died carrying out this particular piece of folly.

 Queen Ranavalona faced several plots and at least one serious coup attempt; as she grew more paranoid she forced more people to undergo the notorious tangena test: eating three pieces of chicken skin before swallowing a poisonous nut that caused the victim to vomit (if it did not actually poison them, which it often did). If all three pieces were not found in the vomit, the victim was executed.

 Having encouraged Christianity at the start of her reign, Queen Ranavalona changed policy and instituted ruthless persecution of native Christians. She survived all plots against her and died in her bed.

 King Leopold II of Belgium (1865–1909)

Leopold's place in this list results not from his rule in Belgium, but from the crimes committed in the enormous kingdom he carved out for himself in Congo. He obtained the territory by international agreement and named it the Congo Free State; it was not a Belgian colony, but the king's personal fiefdom.

 The CFS was presented to the world as a model of liberty and prosperity, devoted to the elimination of slavery. Only gradually did the world learn that it was in fact a slave state in which the Congolese were ruled by terror.

 As Leopold raked in the riches from Congo's enormous reserves of copper, ivory and rubber, the Congolese were forced to work by wholesale mutilation of their wives and children, usually by chopping off their hands or feet. Mutilation was also widely used as a punishment for workers who ran away or collected less than their quota.

An investigation by the British consular official Roger Casement revealed that the Belgian Force Publique regarded the Congolese as little more than animals to be killed for sport. The king fought a high-profile legal battle to prevent details of his regime in Congo from being made public, and it took an international campaign to force him to hand Congo over to the Belgian government.

 Leopold's name is forever associated with the Congolese reign of terror, and that alone justifies his inclusion in this list.