The period between Nero's death on June 9, AD68 and the death of Aulus Vitellius on December 22, AD69 is commonly referred to as the Year of the Four Emperors.

Nero
The direct line of events that caused Nero's downfall can be traced to late AD67 when Gaius Julius Vindex, who is thought possibly to have been the governor of Gallia Lugdunesis, wrote a letter to all the western governors asking them to rebel against Nero on grounds of his matricide and Hellenistic behaviour. A number of governors duly forwarded the letter to Nero who was on his concert tour of Greece but he took no action as Vindex did not control any troops and he was confident that the Rhine legions could handle any uprisings.

In March of AD68, probably timed to coincide with the death of both Julius Caesar on the 15th and Agrippina on the 19th, Vindex started a rebellion supporting Sulpicius Galba, the governor of Tarmconensis Spain, as Emperor. Nero was still not overly concerned but this changed when Galba, who did have a legion, gave his support to Vindex in early April. Although this was a serious development Nero would have still expected the Rhine legions under Verginius Rufus to contain the rebellion.

In May of AD68 Vindex and Rufus meet and had a discussion, the intentions of which are unknown, during its course  Rufus’s legions slaughtered Vindex’s supporters (locals he had raised in Gaul) against the wishes of either commander. This is the official version of events though it is highly likely that Rufus remained loyal to Nero and he was doing his job, the subsequent events would have given him a need to obscure this story to preserve his life. Whatever the motives this slaughter to believe his rebellion was over and he committed suicide

 Although the death of Vindex theoretically ended the rebellion it is clear that Nero had lost control of the Empire. By mid May there were three separate challenges to his position.

• The Rhine legions responsible for defeating Vindex supported Rufus as Emperor. Though Rufus refused, Nero had clearly lost the support of these legions. (Although this is disputed by some modern historians).
• The governor of Africa, Clodius Macer, declared himself Emperor and cut off half the grain supply to Rome.
• Galba, still in Spain, maintained his claim for Emperor and was trying to raise support.

Nero's position now seemed very insecure. In late May of AD68 Galba’s agents bribed Nero’s Praetorian Prefect, Nymphidius Sabinus, to remove the Praetorian Guards’ support of Nero with a bribe of 80,000 sesterces per man; this was possibly done without Galba’s knowledge. The Praetorian Guard soon declared for Galba and some of the other armies soon followed suit. The loss of the support of the Praetorian Guard was fatal for Nero and he was forced to flee Rome. On the 8th of June the senate declared Nero a public enemy and declared Galba the new Emperor. Nero committed suicide on the 9th of June.

Galba (June 8 AD68 - 15 January AD69)
Once the senate declared him emperor Galba started making his way from Spain to Rome. His immediate concern was to cement his position and during his long journey to Rome he had a number of issues to deal with

• Verginius Rufus, the commander of the legion of Germania Superior responsible for killing Vindex’s supporters, did not immediately support Galba. Galba recalled him to Rome where he was to face prosecution, causing the Rhine legions to realise they were distrusted.

• Fonteius Capito, the commander of the four legions of Germania Inferior is assassinated either by Galba for revolting or by the legion commanders for failing to revolt.

• Clodius Macer, the governor of Africa, was executed for his rebellion and pretensions to the principate.

• Nymphidius Sabinus made his own bid to be made emperor, the Praetorian Guards however did not support him and he was killed, this however caused a number of the Praetorian Guard to feel their position was insecure.

• When he arrived in Rome a large number of soldiers arrived presenting demands, Galba had all of these men slaughtered

The executions during his trip to Rome caused the people of Rome some trepidation. Galba then proceeded to alienate the Praetorian guard by not paying them their promised donative. The Praetorians were already aware that they were unlikely to gain much favour from Galba as his power was based on the support of his legions. This slight so soon after the death of Nymphidius Sabinus would have made them very uneasy.

When Galba arrived at Rome in October AD68 he then set out to try and balance the budget; he did this by introducing a number of unpopular measures such as cutting the grain dole and setting up a committee of equestrians who tried to collect the 2.2 billion sesterces Nero had squandered in largesse. Galba demanded that those who had received gifts from Nero returned 90% of the amount given to them. In most cases this money had already been spent and this resulted in confiscations and law suits.

On January 1, AD69 the legions were meant to make their customary oath of loyalty to the emperor. Two of the legions of Germania Superior refused, demanding a new emperor. On January 2 the troops of Germania Inferior also rebelled and named the governor of the province Aulus Vitellius as their candidate for Emperor.

Galba suddenly became aware of how precarious his position was. He was 73 years old without a heir, to introduce some stability he needed to name an heir. Galba chose to support Piso Lucinanius, a virtual unknown who did have the support of the senate but did not have any military experience or the support of the people. Galba's long time supporter Salvius Otho disappointed at not being selected as successor, saw that Galba was out of favour and Piso yet to earn favour, he entered negotiations with the disgruntled Praetorians and offered them a large bribe to make him emperor. On the 15th of January the Praetorians murdered both Galba and Piso.

Otho (15 January AD69 - 16 April AD69)
When Otho became Emperor it is likely that he was unaware of the extent of the rebellion on the Rhine. Although he had the support of the Senate, the Praetorian Guards and the people of Rome unless he could stop the Rhine his reign was destined to be very short. 

He initially offered Vitellius a share of the Empire, which was refused, then Otho set about preparing for war. He summoned the Danube legions to his aid and marched north with the Praetorian Guard to try and stop Vitellius's troops from entering Italy, which he was unsuccessful at doing.

Vitellius's army had split into two and Otho defeated the first force at Bedriacum. He then decided to attack the superior second force before the Danubian legions arrived, presumably as he wished to cut them off from reaching Rome. This fight soon turned against Otho and he committed suicide to prevent further bloodshed.

Aulus Vitellius (17 April AD69 - 22 December AD69)