Pelagius was a contemporary of Augustine who taught that no one is born with a sin nature and our human intellect, wisdom and willpower are sufficient to overcome sin should it appear.  Pelagianism was condemned at the Council of Ephesus in 431 AD.  Semi-Pelagianism, a slight variation, taught that, in addition to human intellect, wisdom and willpower, God may be useful for overcoming particularly virulent types of sin.  Semi-Pelagianism was condemned at the Council of Orange in 529 AD.
Calvinists label all Christians of Traditional Southern Baptist, Wesleyan/Arminian persuasion as heretics promulgating some form of Pelagianism.  Wesleyan/Arminians return the sentiment by accusing Calvinists of blaspheming the Holy Spirit by assigning all Satan’s evil to God per the 1646 Westminster Confession – God from all eternity, did, by the most wise and holy counsel of His own will, freely, and unchangeably ordain whatsoever comes to pass.  This great theological controversy has split churches, destroyed Christian fellowship and had a negative impact on evangelism and discipleship.  Analysis reveals no point of compromise between Calvinism and Arminianism; if one is correct, the other is a complete lie.
Arminius permitted accusations of Pelagianism to circulate for two years before responding to them in his Apology Against Thirty-one Defamatory Articles.  He was not Pelagian, for he believed profoundly in the sin nature.  He believed we are fallen and cannot, unaided by prevenient grace, exercise our capacity of free will in choosing righteousness.  He said, “In this state, the freewill of man towards the true good is not only wounded, maimed…but it is also imprisoned, destroyed and lost.”  He goes on to write, “The mind, in this state, is dark, destitute of the saving knowledge of God, and, according to the Apostle, incapable of those things which belong to the Spirit of God.  For the animal man has no perception of the things of the Spirit of God.”  Further he writes, “Exactly correspondent to this darkness of the mind, and perverseness of the heart, is the utter weakness of all the powers to perform that which is truly good, and to omit the perpetration of that which is evil.”  Finally he says, “It follows that our will is not free from the first fall; that is, it is not free to do good, unless it be made free by the Son through His Spirit.”  The teachings of Arminius were solidified by the Five Articles of Remonstrance in 1610 and further refined by John Wesley.
(See also sections 4.9 and 4.10 of Theology Corner)