The Senatus-Prætoriani Interregis was a strict and formal (though at the same time fragile) agreement lasting from 943(190) to 948(195) between the Senate and the Praetorian guard following Cæsar Commodus' execution. The Interregis was declared upon the execution of Commodus and most of his supporters in the Senate and military, but it was already known that various elements would take the opportunity to seize power. These elements came mostly from the east where the highest concentration of imperial legions were stationed. Most were dealt with quietly, however three greatly marked the Interregis.
Syrianus' March to the Ground
Lucius Clodius Syrianus was one such element. A Victor in the Parthian conquest, and having celebrated his Triumph. Syrianus was the imperial governor of Syria (appointed by Commondus) at the time of Commodus' trial and sentence (October 31 943(190)). Once he received news of Commodus' dethronement, he convinced his legions to declare him Cæsar and immediately with four legions set off by rail for Italia, soliciting popular support at his few vaposcurrs' stops, leaving behind 10 legions under the command of what he considered trustworthy legates and tribunes to gain the support or submission of the Parthian legions. Syrianus encountered no resisting armies in Asia, but upon entering Greece, the legions loyal to the Senate-Praetorian government began intensive operations to cut Syrianus off from all supplies and communication while marching. Vaposcurr lines were destroyed and trade caravans redirected around Syrianus's armies, forcing Syrianus to march. Unable to procure ships to cross the Strait of Otranto to Italia, Syrianus had to go the long way while already ahead of him the vaposcurr lines and engines were put out of business. Within two weeks Syrianus resorted to besieging nearby cities for food and supplies, unable to persuade them to his cause. These were all schemes of the Senate to slow Syrianus down while deputies and spies wore down the troops' support for Syrianus. By winter Syrianus' armies were bereft of resources and were deserting either to the legitimate government or just disappearing. At the same time a series of short battles between Syrianus' Syria legions and the Parthian legions brought the rebels back to the Republic's side. All progress for Syrianus halted in middle Dalmatia and on January 13th 944(191) Syrianus was killed by his council and the remains of his legions surrendered to the Senate. The rebellion became known as Descendsum Caducum Syriani in reference to the protracted difficulties he experienced.
Junius' Light Civil War
Varius Albinus Junius was another. A popular legate in Britannia, in 946(193) Junius led a revolt with his legion and had the Imperial governor killed. Not wanting to make the mistakes of Syrianus; Junius, instead of consolidating Britannia, immediately crossed into Gaul and secured a vaposcurr (with just his one legion and auxiliaries). He headed directly to Rome and managed to hold the Senate hostage after a short battle in the streets with the Praetorians, whom in this era were not finely trained in open combat. After less than a week reinforcements from Germania Superior arrived and quickly defeated Junius' army again within the city limits. Junius had brought such a small force as he had been hoping to scare the Senate into declaring him Cæsar, torturing and killing several senators. Not enough were persuaded however and most knew his strength would be fleeting. Junius was captured and put to death. His decision to make a lightning fast approach on the capital while not having any real plans to further secure his military position became known as 'bella civis expeditum'. When the Senate resumed and the replacements for the killed arrived, reforms to the empire's constitution were made to better control the military, trying to emphasise that it was merely a tool of the government. The Praetorian Guard had been killed almost to a man in Junius' war. The Guard was reformed and its training programmes were revised to higher standards and its ranks expanded, but this did not properly address the growing political influence of the Guard itself.
The Dexion Betrayal
Camillus Camilius Dexion was the last trouble experienced during the Interregis. As a Praetorian he was a survivor of Junius' massacre (how is not clear), and after Junius' defeat he was made sole prefect of the Guard in 946(193). At first playing the role acceptably, in September 946(193) Dexion, with support from his Guard, began making increasing demands of the Senate regarding payment and governing power, even going so far as to stipulate that his tribunes be in line for succeeding the cæsarian provincial governors. The situation came to a head when in 948(195) the Senator Caricus Tiberius exposed Dexion's plans to assassinate much of the Senate and declare himself Cæsar. Dexion was tried but the tribunal was convinced by Caricus not to execute him, stating that death had become too at home in the Senate house in the last five years. Dexion was exiled to Crete, and soon after Caricus was elected Cæsar, ending the Interregis. It was also at this point that the Constitution of the Roman Empire was first put on paper.