303-312 AD — 10 years — last and most terrible of government persecutions of early Church.

You shall have affliction 10 days (can be translated as periods or years) — Revelation 2:10

Pray for those in authority that we may lead a tranquil, undisturbed existence in true worship & devotion — 1 Tim.2:2

In 292 AD, Emperor Diocletian divides the empire between himself and three other military generals, none of them now residing in Rome. He delegates the West to his senior co-Emperor (Augustus) Maximian, who rules from the administrative centres of Milan (in Italy) and Trier (in Germany). Maximian then sub-delegates Gaul, Spain and Britain to his junior sub-Emperor (Caesar) Constantius. Diocletian himself takes Thrace, Egypt and Asia in the East, ruling from his capital Nicomedia (modern day Izmit in Turkey). Galerius (his Caesar son-in-law) is stationed on the Danube.

Meanwhile, Eusebius records how rulers of the Church were being courted and honoured. Vast multitudes were flocking to the religion of Christ, with spacious and splendid churches being erected in the place of the humbler buildings of earlier days. Though he also bewails the falling from ancient fervour "by reason of excessive liberty".

While wintering together in 302, Galerius urges Diocletian to institute a universal persecution of all Christians. Legislation is passed ordering the destruction of all Christian scriptures and churches, and commences, in February 303, with the demolishing of the cathedral in Nicomedia. Leaders are imprisoned and tortured. A whole town who declared themselves "Christians" were massacred. This government sponsored persecution lasts exactly 10 years, finishing with the edict of Milan in January 313.
YearWestern AugustusWestern CaesarEastern AugustusEastern Caesar
303 MaximianConstantiusDiocletian (in charge)Galerius
305 Maximian, at Diocletian's behest, retires, handing over to Constantius. Severus (old army friend of Galerius) is appointed as Western Caesar by Diocletian, to the dismay of Maxentius (Maximian's son).SeverusDiocletian, suffering from a debilitating sickness, retires.
Galerius takes over.
Maximinus (Galerius's nephew)
306 Constantius dies in York, Britain. Severus takes over in Milan. Constantine (Constantius's son) is nominated by troops to be Augustus, but accepts post of Caesar following Galerius's insistence on Severus. Maxentius is nominated by the army in Rome as overall Emperor.
ConstantineGaleriusMaximinus
307 Severus marches on Rome. Maximian retakes title of Augustus and with army in Rome captures Severus. Severus imprisoned then executed when Galerius unsuccessfully invades Rome. Meanwhile, Constantine now marries Maximian's daughter Fausta, becomes Augustus in Gaul Spain and Britain. GaleriusMaximinus
308 Maxentius names himself Augustus in Italy and Africa with Constantine ruling Gaul Spain and Britain. Maximian attempts to depose his son, but fails. Galerius appoints Licinius to be Western Augustus. GaleriusMaximinus
310 Licinius (in absentia) over Constantine (and Maxentius). Maximian declares himself Augustus in Gaul — defeated by Constantine — at Constantine's urging, commits suicide. Maximinus with Galerius now very ill 
311 Maxentius declares war on Constantine. Constantine forges Eastern alliance with Licinius, offers sister Constantia in marriage to seal treaty. Licinius (and Maximinus).
Galerius dies
 
312 Maxentius defeated by Constantine in Rome, drowns. In Constantine's battle for Rome, his soldiers had famously had the "anointed" Chi-Rho symbol, the first two letters for Christ (in Greek) emblazoned on their shields & helmets. Licinius (and Maximinus)

In December Diocletian dies.

 
In January, 313 Constantine and Licinius sign Treaty of Milan that restores confiscated properties to Christian congregations. Licinius marries Constantia — seals treaty. Maximinus attacks Licinius in battle — is defeated and dies. In 324, after 11 years of east versus west intrigues, battles, Licinius defeated by Constantine & executed. Constantine now fully in charge.