Constantine Comes to Power
While the persecution was being carried out in the East where the numbers of Christians were much greater, the Western Christians experienced very little pressure. Under Constantius (the father of Constantine) some church buildings were destroyed in Britain, Gaul, and Spain - that was the extent of persecution - there is no evidence that any Christian was executed. Constantine's father, though not a Christian was married to a Christian woman. In addition, Constantine's half sister was named Anastasia (anastasis - Greek for "resurrection"). This is more indication of Christian influence in the household of Constantius.

The details are far more complicated with the empire being led by a Tetrarchy (an Augustus ruler with a Caesar under him in both the East and in the West. In addition, there were several struggles (like civil war) for control. A simple overview:
When Constantius died in 306 his military proclaimed his son Constantine to be the new emperor in the West.
306 - Constantius died, his son Constantine replaced him
310 - Maximian rebels against Constantine and fails
311 - Galerius died, Maximin Daia replaces him in the East
311 - Maximin and Licinius fight for control in the East
311 - Constantine and Licinius form an alliance

In 312 Constantine engaged Maxentius (son of Maximian) in battle for sole rulership in the West at the famous "Battle of the Milvian Bridge." According to Lactantius (the date given later by Eusebius indicates an earlier battle) Constantine had a vision of a cross in the sky and heard a voice saying something like, "Go, and in this symbol, conquer." In the battle Maxentius drowns in the Tiber river attempting to retreat. Though the details are sketchy and not easy to fully reconcile, it appears that Constantine knew enough of Christianity to believe that his vision had been given to him by the God of the Christians, that he was chosen (or destined) by this God to rule the empire, and it was the beginning of his embrace of Christian faith. We will discuss the faith of Constantine in more detail later.

This battle leaves Constantine as the sole leader of the empire in the West. In 313 Constantine and Licinius sign the Treaty of Milan. This treaty marks an historic moment for the Christian faith. It is decreed that all Roman citizens would have religious freedom - the ability to worship (any "god" they wanted) without interference from the empire. While this was NOT Constantine making Christianity the official religion, it effectively put an end to the persecution of Christians.

This is an image of the Battle of the Milvian Bridge from the Arch of Constantine in Rome, commissioned by the Senate fairly soon after the battle.
Source: Wikipedia

This image was erected by the Roman senate under the banner of the Unconquered Sun. Many people point to examples like this and others where Constantine (or references made by/toward him) mention the Sun. It is quite important to remember that Constantine and the early Christians utilized aspects of pagan images, but at the same time clearly pointed to Christ as their "Lord."

Constantine and Licinius also entered something of a truce, putting an end to the leadership strife that had existed for the previous 20-25 years. Licinius later begins to persecute Christians in the east while Constantine was showing them favor in the west. This led to a final conflict, when in 324 AD the truce ended, Constantine defeated Licinius in battle and became the sole ruler of the Roman empire.

The Church Divided
As with prior times of persecution, schisms developed in the church regarding how to deal with those who had "lapsed" in their faith (see the discussion on Second Repentance, Second Century). In the East, where the persecution had been most severe, there tended to be a more lenient treatment of those who had failed in some way. In the West, especially in North Africa, a more strident view held the day. While some bishops had been able to satisfy the authorities with copies of Gnostic works to be burned, in some parts of North Africa handing over any document to be burned (even a medical book) was considered apostasy - even the appearance of cooperating was to deny the faith.

Many try to blame Constantine for the difficulties Christianity encountered in the fourth century. Right now we will only mention the first two "heretics," Donatus and Arius. Constantine tried help with both of these problems. He was astonished with the divisions he found within the faith.

British Classical Historian Robin Lane Fox offers a balanced and nuanced presentation: