The English Civil War
The war between the Crown (Cavaliers) and Parliament (Roundheads: closely cropped hair) was caused by an uprising of Irish Catholics after the massacre of Ulster in 1641, and the arrest of John Pym (one of the king's parliamentary opponents).

Decisive factors in the war were the intervention of Scotland in 1643 and the new Parliamentary army under Oliver Cromwell.

After Cromwell's victory charges of treason were brought against king Charles I. The trial was followed by the execution of the king and the abolition of the monarchy.

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From infoplease
The English Civil War